fix(clients): inline Go gosec directive and strip IPv6 brackets in Python authority split

feat(gateway): add machine FQDN to self-signed cert SANs
Best-effort resolve the host FQDN via Dns.GetHostEntry and add it as a DNS SAN when it differs (OrdinalIgnoreCase) from the short machine name and "localhost". SocketException / ArgumentException are caught and silently skipped so cert generation remains robust when DNS is absent.
2026-06-01 07:57:22 -04:00 · 2026-06-01 07:52:48 -04:00 · 2026-06-01 07:45:15 -04:00 · 2026-06-01 07:43:13 -04:00 · 2026-06-01 07:37:27 -04:00 · 2026-06-01 07:30:26 -04:00
49 changed files with 3658 additions and 63 deletions
@@ -107,6 +107,7 @@ public sealed class MxGatewayClientOptions
    public required string ApiKey { get; init; }
    public bool UseTls { get; init; }
    public string? CaCertificatePath { get; init; }
+    public bool RequireCertificateValidation { get; init; }
    public string? ServerNameOverride { get; init; }
    public TimeSpan ConnectTimeout { get; init; } = TimeSpan.FromSeconds(10);
    public TimeSpan DefaultCallTimeout { get; init; } = TimeSpan.FromSeconds(30);
@@ -124,6 +125,24 @@ or subscription changes because those calls can partially succeed in MXAccess.
 API key may be loaded from `MXGATEWAY_API_KEY` by the CLI, not implicitly by the
 library constructor unless a helper explicitly says it does that.

+### TLS trust posture
+
+The gateway can serve a self-signed certificate it generates itself (it has no
+PKI). To make that usable, TLS is **lenient by default**: when `UseTls` is set
+and `CaCertificatePath` is empty, `CreateHttpHandler` installs a
+`RemoteCertificateValidationCallback` that returns `true`, so the gateway's
+self-signed certificate is accepted without verification.
+
+To verify the gateway instead:
+
+- set `CaCertificatePath` to pin a CA — validated via a `CustomRootTrust`
+  `X509Chain` against that root, and the callback additionally rejects a
+  hostname/SAN mismatch (`RemoteCertificateNameMismatch`); or
+- set `RequireCertificateValidation` to `true` to keep the default OS/system-trust
+  verification on a connection with no pinned CA.
+
+Pinning a CA always wins over the lenient default.
+
 ## Auth Interceptor

 Use a gRPC call credentials/interceptor layer to attach:
@@ -287,6 +287,17 @@ Use TLS options for a secured gateway:
 dotnet run --project clients/dotnet/ZB.MOM.WW.MxGateway.Client.Cli -- smoke --endpoint https://ZB.MOM.WW.MxGateway.example.local:5001 --tls --ca-file C:\certs\mxgateway-ca.pem --server-name ZB.MOM.WW.MxGateway.example.local --api-key-env MXGATEWAY_API_KEY --item Area001.Pump001.Speed --json
 ```

+### TLS trust
+
+The gateway can auto-generate its own self-signed certificate (it has no PKI), so
+the client is **lenient by default**: a TLS connection (`UseTls` / `--tls`) with
+no pinned CA accepts whatever certificate the gateway presents. To verify
+instead, pin a CA with `CaCertificatePath` / `--ca-file` (this path also enforces
+the certificate hostname/SAN match), or set `RequireCertificateValidation` to
+force OS/system-trust verification without pinning. Use `ServerNameOverride` /
+`--server-name` when the dialed host differs from the certificate SAN. See
+[Gateway Configuration](../../docs/GatewayConfiguration.md#automatic-self-signed-certificate).
+
 ## Integration Checks

 Run live checks only when a gateway and MXAccess-backed worker are available:
@@ -0,0 +1,85 @@
+using System.Net.Http;
+using System.Net.Security;
+using ZB.MOM.WW.MxGateway.Client;
+
+namespace ZB.MOM.WW.MxGateway.Client.Tests;
+
+public sealed class MxGatewayClientTlsHandlerTests
+{
+    /// <summary>
+    /// Verifies that when TLS is used with no pinned CA and RequireCertificateValidation is false (default),
+    /// the handler installs an accept-all callback so the gateway's self-signed cert is trusted.
+    /// The callback must return true regardless of chain errors.
+    /// </summary>
+    [Fact]
+    public void Handler_SkipsVerification_WhenTlsAndNoCaPinned()
+    {
+        MxGatewayClientOptions options = new()
+        {
+            Endpoint = new Uri("https://localhost:5120"),
+            ApiKey = "k",
+            UseTls = true,
+        };
+        using SocketsHttpHandler handler = MxGatewayClient.CreateHttpHandlerForTests(options);
+        Assert.NotNull(handler.SslOptions.RemoteCertificateValidationCallback);
+        Assert.True(handler.SslOptions.RemoteCertificateValidationCallback!(null!, null!, null, SslPolicyErrors.RemoteCertificateChainErrors));
+    }
+
+    /// <summary>
+    /// Verifies that when RequireCertificateValidation is true, the callback is left null
+    /// so the OS trust store performs validation.
+    /// </summary>
+    [Fact]
+    public void Handler_KeepsDefaultVerification_WhenRequireCertificateValidation()
+    {
+        MxGatewayClientOptions options = new()
+        {
+            Endpoint = new Uri("https://localhost:5120"),
+            ApiKey = "k",
+            UseTls = true,
+            RequireCertificateValidation = true,
+        };
+        using SocketsHttpHandler handler = MxGatewayClient.CreateHttpHandlerForTests(options);
+        Assert.Null(handler.SslOptions.RemoteCertificateValidationCallback);
+    }
+}
+
+public sealed class GalaxyRepositoryClientTlsHandlerTests
+{
+    /// <summary>
+    /// Verifies that when TLS is used with no pinned CA and RequireCertificateValidation is false (default),
+    /// the Galaxy client handler installs an accept-all callback so the gateway's self-signed cert is trusted.
+    /// The callback must return true regardless of chain errors.
+    /// </summary>
+    [Fact]
+    public void Handler_SkipsVerification_WhenTlsAndNoCaPinned()
+    {
+        MxGatewayClientOptions options = new()
+        {
+            Endpoint = new Uri("https://localhost:5120"),
+            ApiKey = "k",
+            UseTls = true,
+        };
+        using SocketsHttpHandler handler = GalaxyRepositoryClient.CreateHttpHandlerForTests(options);
+        Assert.NotNull(handler.SslOptions.RemoteCertificateValidationCallback);
+        Assert.True(handler.SslOptions.RemoteCertificateValidationCallback!(null!, null!, null, SslPolicyErrors.RemoteCertificateChainErrors));
+    }
+
+    /// <summary>
+    /// Verifies that when RequireCertificateValidation is true, the Galaxy client callback is left null
+    /// so the OS trust store performs validation.
+    /// </summary>
+    [Fact]
+    public void Handler_KeepsDefaultVerification_WhenRequireCertificateValidation()
+    {
+        MxGatewayClientOptions options = new()
+        {
+            Endpoint = new Uri("https://localhost:5120"),
+            ApiKey = "k",
+            UseTls = true,
+            RequireCertificateValidation = true,
+        };
+        using SocketsHttpHandler handler = GalaxyRepositoryClient.CreateHttpHandlerForTests(options);
+        Assert.Null(handler.SslOptions.RemoteCertificateValidationCallback);
+    }
+}
@@ -490,7 +490,10 @@ public sealed class GalaxyRepositoryClient : IAsyncDisposable
            .ConfigureAwait(false);
    }

-    private static HttpMessageHandler CreateHttpHandler(MxGatewayClientOptions options)
+    private static HttpMessageHandler CreateHttpHandler(MxGatewayClientOptions options) =>
+        CreateHttpHandlerForTests(options);
+
+    internal static SocketsHttpHandler CreateHttpHandlerForTests(MxGatewayClientOptions options)
    {
        SocketsHttpHandler handler = new()
        {
@@ -510,6 +513,11 @@ public sealed class GalaxyRepositoryClient : IAsyncDisposable
                X509Certificate2 trustedRoot = X509CertificateLoader.LoadCertificateFromFile(options.CaCertificatePath);
                handler.SslOptions.RemoteCertificateValidationCallback = (_, certificate, chain, errors) =>
                {
+                    if ((errors & System.Net.Security.SslPolicyErrors.RemoteCertificateNameMismatch) != 0)
+                    {
+                        return false;
+                    }
+
                    if (certificate is null)
                    {
                        return false;
@@ -525,6 +533,10 @@ public sealed class GalaxyRepositoryClient : IAsyncDisposable
                    return customChain.Build(certificateToValidate);
                };
            }
+            else if (!options.RequireCertificateValidation)
+            {
+                handler.SslOptions.RemoteCertificateValidationCallback = (_, _, _, _) => true;
+            }
        }

        return handler;
@@ -315,7 +315,10 @@ public sealed class MxGatewayClient : IAsyncDisposable
            .ConfigureAwait(false);
    }

-    private static HttpMessageHandler CreateHttpHandler(MxGatewayClientOptions options)
+    private static HttpMessageHandler CreateHttpHandler(MxGatewayClientOptions options) =>
+        CreateHttpHandlerForTests(options);
+
+    internal static SocketsHttpHandler CreateHttpHandlerForTests(MxGatewayClientOptions options)
    {
        SocketsHttpHandler handler = new()
        {
@@ -335,6 +338,11 @@ public sealed class MxGatewayClient : IAsyncDisposable
                X509Certificate2 trustedRoot = X509CertificateLoader.LoadCertificateFromFile(options.CaCertificatePath);
                handler.SslOptions.RemoteCertificateValidationCallback = (_, certificate, chain, errors) =>
                {
+                    if ((errors & System.Net.Security.SslPolicyErrors.RemoteCertificateNameMismatch) != 0)
+                    {
+                        return false;
+                    }
+
                    if (certificate is null)
                    {
                        return false;
@@ -350,6 +358,10 @@ public sealed class MxGatewayClient : IAsyncDisposable
                    return customChain.Build(certificateToValidate);
                };
            }
+            else if (!options.RequireCertificateValidation)
+            {
+                handler.SslOptions.RemoteCertificateValidationCallback = (_, _, _, _) => true;
+            }
        }

        return handler;
@@ -27,6 +27,14 @@ public sealed class MxGatewayClientOptions
    /// </summary>
    public string? CaCertificatePath { get; init; }

+    /// <summary>
+    /// When true, TLS connections without a pinned <see cref="CaCertificatePath"/>
+    /// use the OS trust store. When false (default), the gateway certificate is
+    /// accepted without verification — appropriate for this internal tool's
+    /// auto-generated self-signed certificate. Pinning a CA always verifies.
+    /// </summary>
+    public bool RequireCertificateValidation { get; init; }
+
    /// <summary>
    /// Gets the server name override for SNI during TLS handshake.
    /// </summary>
@@ -27,4 +27,10 @@
    <None Include="..\README.md" Pack="true" PackagePath="\" />
  </ItemGroup>

+  <ItemGroup>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
+      <_Parameter1>ZB.MOM.WW.MxGateway.Client.Tests</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
+
 </Project>
@@ -104,6 +104,23 @@ Support:
 - `credentials.NewClientTLSFromFile`,
 - custom `tls.Config` for advanced callers.

+### Trust posture
+
+The gateway can serve a self-signed certificate it generates itself (it has no
+PKI). To make that usable, TLS is **lenient by default**: when `Plaintext` is
+`false` and no `CACertFile`/`TLSConfig`/`TransportCredentials` is supplied,
+`buildCredentials` dials with `tls.Config{InsecureSkipVerify: true}` (carrying
+`ServerNameOverride` as the SNI when set), so the gateway's self-signed
+certificate is accepted without verification.
+
+To verify the gateway instead:
+
+- set `CACertFile` to pin a CA (full verification against that root), or
+- set `RequireCertificateValidation: true` to verify against the OS/system trust
+  roots without pinning.
+
+Pinning a CA always wins over the lenient default.
+
 ## Streaming

 `Events(ctx)` should return a receive channel of:
@@ -75,6 +75,14 @@ client, err := mxgateway.Dial(ctx, mxgateway.Options{
 })
 ```

+The gateway can auto-generate its own self-signed certificate (it has no PKI), so
+the client is **lenient by default**: a TLS connection (`Plaintext: false`) with
+no `CACertFile`/`TLSConfig` accepts whatever certificate the gateway presents
+(`InsecureSkipVerify`, with `ServerNameOverride` as the SNI when set). To verify
+instead, set `CACertFile` to pin a CA, or set `RequireCertificateValidation:
+true` to verify against the OS/system trust roots without pinning. See
+[Gateway Configuration](../../docs/GatewayConfiguration.md#automatic-self-signed-certificate).
+
 `Client.OpenSession` returns a `Session` with helpers for `Register`,
 `AddItem`, `AddItem2`, `Advise`, `Write`, `Events`, and `Close`. Prefer
 `SubscribeEvents` or `SubscribeEventsAfter` for long-running streams because the
@@ -222,10 +222,22 @@ func resolveTransportCredentials(opts Options) (credentials.TransportCredentials
 		return credentials.NewTLS(cfg), nil
 	}

-	return credentials.NewTLS(&tls.Config{
-		MinVersion: tls.VersionTLS12,
-		ServerName: opts.ServerNameOverride,
-	}), nil
+	return credentials.NewTLS(tlsConfigForOptions(opts)), nil
+}
+
+// tlsConfigForOptions returns the *tls.Config for the no-CA, no-custom-config TLS path.
+// It returns nil when the caller should use a different credentials path (CA file or custom TLSConfig).
+// Exposed as an internal helper so unit tests can assert the InsecureSkipVerify posture.
+func tlsConfigForOptions(opts Options) *tls.Config {
+	// CA file and custom TLSConfig take their own paths in resolveTransportCredentials.
+	if opts.CACertFile != "" || opts.TLSConfig != nil {
+		return nil
+	}
+	return &tls.Config{
+		MinVersion:         tls.VersionTLS12,
+		ServerName:         opts.ServerNameOverride,
+		InsecureSkipVerify: !opts.RequireCertificateValidation, //nolint:gosec // internal tool; self-signed gateway cert expected; opt-in strict via RequireCertificateValidation
+	}
 }

 // OpenSessionOptions describes fields used to create an OpenSessionRequest.
@@ -0,0 +1,59 @@
+package mxgateway
+
+import (
+	"crypto/tls"
+	"testing"
+)
+
+// tlsConfigFromOptions is the internal helper under test.
+// It extracts the *tls.Config from the no-CA TLS path of resolveTransportCredentials.
+// We exercise it directly to avoid needing a real dial target.
+
+func TestTLSInsecureSkipVerify_DefaultTrue(t *testing.T) {
+	cfg := tlsConfigForOptions(Options{
+		Endpoint: "localhost:5120",
+	})
+	if cfg == nil {
+		t.Fatal("expected non-nil tls.Config")
+	}
+	if !cfg.InsecureSkipVerify {
+		t.Error("InsecureSkipVerify should be true by default when no CA is pinned")
+	}
+}
+
+func TestTLSInsecureSkipVerify_FalseWhenRequireCertificateValidation(t *testing.T) {
+	cfg := tlsConfigForOptions(Options{
+		Endpoint:                     "localhost:5120",
+		RequireCertificateValidation: true,
+	})
+	if cfg == nil {
+		t.Fatal("expected non-nil tls.Config")
+	}
+	if cfg.InsecureSkipVerify {
+		t.Error("InsecureSkipVerify should be false when RequireCertificateValidation is true")
+	}
+}
+
+func TestTLSInsecureSkipVerify_FalseWhenCACertFileSet(t *testing.T) {
+	// When a CA file is pinned, the CA-verification path is taken instead.
+	// tlsConfigForOptions should return nil (the CA path does not use our helper).
+	cfg := tlsConfigForOptions(Options{
+		Endpoint:   "localhost:5120",
+		CACertFile: "/some/ca.pem",
+	})
+	if cfg != nil {
+		t.Error("expected nil tls.Config when CACertFile is set (CA path taken)")
+	}
+}
+
+func TestTLSInsecureSkipVerify_FalseWhenCustomTLSConfig(t *testing.T) {
+	// When TLSConfig is supplied explicitly, our default skip-verify must not overwrite it.
+	custom := &tls.Config{MinVersion: tls.VersionTLS13}
+	cfg := tlsConfigForOptions(Options{
+		Endpoint:  "localhost:5120",
+		TLSConfig: custom,
+	})
+	if cfg != nil {
+		t.Error("expected nil tls.Config when TLSConfig is already set (custom config path taken)")
+	}
+}
@@ -34,6 +34,10 @@ type Options struct {
 	TransportCredentials credentials.TransportCredentials
 	// DialOptions are appended to the gRPC dial options after the defaults.
 	DialOptions []grpc.DialOption
+	// RequireCertificateValidation forces TLS certificate verification even when
+	// no CACertFile is pinned. Default false: the gateway's self-signed cert is
+	// accepted without verification (internal-tool posture).
+	RequireCertificateValidation bool
 }

 // BrowseChildrenOptions configures lazy Galaxy hierarchy walks performed by
@@ -112,6 +112,23 @@ Support:
 - custom CA certificate file,
 - server name override for test environments.

+### Trust posture
+
+The gateway can serve a self-signed certificate it generates itself (it has no
+PKI). To make that usable, TLS is **lenient by default**: when the channel is not
+plaintext and no `caCertificatePath` is set, the client builds
+`GrpcSslContexts.forClient().trustManager(InsecureTrustManagerFactory.INSTANCE)`
+(grpc-netty-shaded), so the gateway's self-signed certificate is accepted without
+verification.
+
+To verify the gateway instead:
+
+- set `caCertificatePath` to pin a CA (full verification against that root), or
+- set `requireCertificateValidation` to `true` to verify against the JVM trust
+  store without pinning.
+
+Pinning a CA always wins over the lenient default.
+
 ## Streaming

 Support both:
@@ -57,6 +57,16 @@ try (MxGatewayClient client = MxGatewayClient.connect(options);
 }
 ```

+The gateway can auto-generate its own self-signed certificate (it has no PKI), so
+the client is **lenient by default**: a TLS connection (`plaintext(false)`) with
+no `caCertificatePath` accepts whatever certificate the gateway presents (via
+grpc-netty-shaded's `InsecureTrustManagerFactory`). To verify instead, set
+`caCertificatePath` to pin a CA, or set `requireCertificateValidation(true)` to
+verify against the JVM trust store without pinning. Use `serverNameOverride` /
+`--server-name-override` when the dialed host differs from the certificate SAN.
+See
+[Gateway Configuration](../../docs/GatewayConfiguration.md#automatic-self-signed-certificate).
+
 Use `rawBlockingStub`, `rawFutureStub`, `rawAsyncStub`, `openSessionRaw`,
 `closeSessionRaw`, `invoke`, and raw session helper methods when tests need the
 underlying protobuf messages. `MxGatewayCommandException` and
@@ -9,6 +9,10 @@ pluginManagement {
    }
 }

+plugins {
+    id 'org.gradle.toolchains.foojay-resolver-convention' version '1.0.0'
+}
+
 dependencyResolutionManagement {
    repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
    repositories {
@@ -384,6 +384,15 @@ public final class MxGatewayClient implements AutoCloseable {
            } catch (SSLException error) {
                throw new MxGatewayException("failed to configure gateway TLS", error);
            }
+        } else if (!options.requireCertificateValidation()) {
+            try {
+                builder.sslContext(GrpcSslContexts.forClient()
+                        .trustManager(io.grpc.netty.shaded.io.netty.handler.ssl.util
+                                .InsecureTrustManagerFactory.INSTANCE)
+                        .build());
+            } catch (SSLException error) {
+                throw new MxGatewayException("failed to configure lenient gateway TLS", error);
+            }
        } else {
            builder.useTransportSecurity();
        }
@@ -393,6 +402,19 @@ public final class MxGatewayClient implements AutoCloseable {
        return builder.build();
    }

+    /**
+     * Package-visible test seam — creates a raw {@link ManagedChannel} from the
+     * given options without attaching auth interceptors. Used by TLS fixture
+     * tests to verify channel construction behaviour without a full
+     * {@link MxGatewayClient} wrapper.
+     *
+     * @param options the client options
+     * @return a new {@link ManagedChannel}
+     */
+    static ManagedChannel createChannelForTests(MxGatewayClientOptions options) {
+        return createChannel(options);
+    }
+
    private <T extends io.grpc.stub.AbstractStub<T>> T withDeadline(T stub) {
        if (options.callTimeout().isNegative()) {
            return stub;
@@ -20,6 +20,7 @@ public final class MxGatewayClientOptions {
    private final String apiKey;
    private final boolean plaintext;
    private final Path caCertificatePath;
+    private final boolean requireCertificateValidation;
    private final String serverNameOverride;
    private final Duration connectTimeout;
    private final Duration callTimeout;
@@ -31,6 +32,7 @@ public final class MxGatewayClientOptions {
        apiKey = builder.apiKey == null ? "" : builder.apiKey;
        plaintext = builder.plaintext;
        caCertificatePath = builder.caCertificatePath;
+        requireCertificateValidation = builder.requireCertificateValidation;
        serverNameOverride = builder.serverNameOverride == null ? "" : builder.serverNameOverride;
        connectTimeout = builder.connectTimeout == null ? DEFAULT_CONNECT_TIMEOUT : builder.connectTimeout;
        callTimeout = builder.callTimeout == null ? DEFAULT_CALL_TIMEOUT : builder.callTimeout;
@@ -95,6 +97,18 @@ public final class MxGatewayClientOptions {
        return caCertificatePath;
    }

+    /**
+     * Returns whether TLS certificate verification is required even when no CA is pinned.
+     * When {@code false} (default), the gateway's self-signed certificate is accepted
+     * without verification. When {@code true}, the OS trust store is used.
+     * Pinning a CA via {@link #caCertificatePath()} always verifies regardless of this flag.
+     *
+     * @return {@code true} if strict certificate verification is required
+     */
+    public boolean requireCertificateValidation() {
+        return requireCertificateValidation;
+    }
+
    /**
     * Returns the TLS server-name override, or an empty string when none was supplied.
     *
@@ -148,6 +162,8 @@ public final class MxGatewayClientOptions {
                + plaintext
                + ", caCertificatePath="
                + caCertificatePath
+                + ", requireCertificateValidation="
+                + requireCertificateValidation
                + ", serverNameOverride='"
                + serverNameOverride
                + '\''
@@ -177,6 +193,7 @@ public final class MxGatewayClientOptions {
        private String apiKey;
        private boolean plaintext;
        private Path caCertificatePath;
+        private boolean requireCertificateValidation;
        private String serverNameOverride;
        private Duration connectTimeout;
        private Duration callTimeout;
@@ -230,6 +247,21 @@ public final class MxGatewayClientOptions {
            return this;
        }

+        /**
+         * When {@code true}, TLS connections without a pinned CA use the OS trust store
+         * and will reject the gateway's self-signed certificate. When {@code false}
+         * (default), the gateway certificate is accepted without verification —
+         * appropriate for this internal tool's auto-generated self-signed certificate.
+         * Pinning a CA via {@link #caCertificatePath(Path)} always verifies.
+         *
+         * @param value {@code true} to require certificate validation, {@code false} to accept any cert
+         * @return this builder
+         */
+        public Builder requireCertificateValidation(boolean value) {
+            requireCertificateValidation = value;
+            return this;
+        }
+
        /**
         * Overrides the TLS server name used during the handshake.
         *
@@ -0,0 +1,198 @@
+package com.zb.mom.ww.mxgateway.client;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import io.grpc.ManagedChannel;
+import io.grpc.Server;
+import io.grpc.StatusRuntimeException;
+import io.grpc.netty.shaded.io.grpc.netty.GrpcSslContexts;
+import io.grpc.netty.shaded.io.grpc.netty.NettyServerBuilder;
+import io.grpc.stub.StreamObserver;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.nio.file.Files;
+import java.security.KeyStore;
+import java.security.PrivateKey;
+import java.security.cert.Certificate;
+import java.security.cert.X509Certificate;
+import java.time.Duration;
+import java.util.Base64;
+import java.util.concurrent.TimeUnit;
+import javax.net.ssl.SSLException;
+import mxaccess_gateway.v1.MxAccessGatewayGrpc;
+import mxaccess_gateway.v1.MxaccessGateway.OpenSessionReply;
+import mxaccess_gateway.v1.MxaccessGateway.OpenSessionRequest;
+import mxaccess_gateway.v1.MxaccessGateway.ProtocolStatus;
+import mxaccess_gateway.v1.MxaccessGateway.ProtocolStatusCode;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Verifies that the Java client connects to a Netty TLS server with a
+ * self-signed certificate when no CA is pinned (lenient default), and that
+ * setting {@code requireCertificateValidation(true)} causes a TLS failure.
+ *
+ * <p>A self-signed certificate is generated using {@code keytool} (always
+ * available in the JDK) to avoid dependencies on internal JDK APIs or
+ * BouncyCastle, and so the test works on all JDK versions used by the project.
+ */
+final class MxGatewayClientTlsTests {
+
+    private Server server;
+    private int port;
+    private File certPemFile;
+    private File keyPemFile;
+    private File keystoreFile;
+
+    @BeforeEach
+    void startTlsServer() throws Exception {
+        keystoreFile = File.createTempFile("gw-test-ks", ".p12");
+        certPemFile = File.createTempFile("gw-test-cert", ".pem");
+        keyPemFile = File.createTempFile("gw-test-key", ".pem");
+
+        // keytool refuses to write to a pre-existing (even empty) file; delete it first.
+        keystoreFile.delete();
+
+        // Use keytool to generate a self-signed PKCS12 keystore.
+        String keytool = ProcessHandle.current().info().command()
+                .map(cmd -> cmd.replace("java", "keytool"))
+                .orElse("keytool");
+        // Fall back to just "keytool" on PATH if the resolved path doesn't exist.
+        if (!new File(keytool).exists()) {
+            keytool = "keytool";
+        }
+        Process p = new ProcessBuilder(
+                keytool,
+                "-genkeypair",
+                "-alias", "server",
+                "-keyalg", "RSA",
+                "-keysize", "2048",
+                "-sigalg", "SHA256withRSA",
+                "-validity", "1",
+                "-dname", "CN=localhost",
+                "-storetype", "PKCS12",
+                "-storepass", "changeit",
+                "-keypass", "changeit",
+                "-keystore", keystoreFile.getAbsolutePath())
+                .redirectErrorStream(true)
+                .start();
+        int exit = p.waitFor();
+        if (exit != 0) {
+            String out = new String(p.getInputStream().readAllBytes());
+            throw new IllegalStateException("keytool failed (exit " + exit + "): " + out);
+        }
+
+        // Export cert and private key from the PKCS12 keystore to PEM files.
+        KeyStore ks = KeyStore.getInstance("PKCS12");
+        try (var is = Files.newInputStream(keystoreFile.toPath())) {
+            ks.load(is, "changeit".toCharArray());
+        }
+        X509Certificate cert = (X509Certificate) ks.getCertificate("server");
+        PrivateKey privateKey = (PrivateKey) ks.getKey("server", "changeit".toCharArray());
+
+        try (FileOutputStream out = new FileOutputStream(certPemFile)) {
+            out.write("-----BEGIN CERTIFICATE-----\n".getBytes());
+            out.write(Base64.getMimeEncoder(64, new byte[]{'\n'}).encode(cert.getEncoded()));
+            out.write("\n-----END CERTIFICATE-----\n".getBytes());
+        }
+        try (FileOutputStream out = new FileOutputStream(keyPemFile)) {
+            out.write("-----BEGIN PRIVATE KEY-----\n".getBytes());
+            out.write(Base64.getMimeEncoder(64, new byte[]{'\n'}).encode(privateKey.getEncoded()));
+            out.write("\n-----END PRIVATE KEY-----\n".getBytes());
+        }
+
+        server = NettyServerBuilder
+                .forAddress(new InetSocketAddress("127.0.0.1", 0))
+                .sslContext(GrpcSslContexts.forServer(certPemFile, keyPemFile).build())
+                .addService(new MinimalGatewayService())
+                .build()
+                .start();
+        port = server.getPort();
+    }
+
+    @AfterEach
+    void stopTlsServer() throws InterruptedException {
+        if (server != null) {
+            server.shutdown();
+            server.awaitTermination(5, TimeUnit.SECONDS);
+        }
+        if (certPemFile != null) {
+            certPemFile.delete();
+        }
+        if (keyPemFile != null) {
+            keyPemFile.delete();
+        }
+        if (keystoreFile != null) {
+            keystoreFile.delete();
+        }
+    }
+
+    @Test
+    void connectsToSelfSignedServer_WhenRequireCertificateValidationIsFalse() throws SSLException {
+        // Default options — requireCertificateValidation defaults to false.
+        MxGatewayClientOptions options = MxGatewayClientOptions.builder()
+                .endpoint("127.0.0.1:" + port)
+                .apiKey("test-key")
+                .connectTimeout(Duration.ofSeconds(5))
+                .callTimeout(Duration.ofSeconds(5))
+                .build();
+
+        ManagedChannel channel = MxGatewayClient.createChannelForTests(options);
+        try {
+            MxAccessGatewayGrpc.MxAccessGatewayBlockingStub stub =
+                    MxAccessGatewayGrpc.newBlockingStub(channel);
+            OpenSessionReply reply = stub.openSession(
+                    OpenSessionRequest.newBuilder()
+                            .setClientSessionName("tls-test")
+                            .build());
+            assertTrue(reply.getProtocolStatus().getCode()
+                    == ProtocolStatusCode.PROTOCOL_STATUS_CODE_OK);
+        } finally {
+            channel.shutdownNow();
+        }
+    }
+
+    @Test
+    void failsToConnect_WhenRequireCertificateValidationIsTrue() throws SSLException {
+        MxGatewayClientOptions options = MxGatewayClientOptions.builder()
+                .endpoint("127.0.0.1:" + port)
+                .apiKey("test-key")
+                .requireCertificateValidation(true)
+                .connectTimeout(Duration.ofSeconds(5))
+                .callTimeout(Duration.ofSeconds(5))
+                .build();
+
+        ManagedChannel channel = MxGatewayClient.createChannelForTests(options);
+        try {
+            MxAccessGatewayGrpc.MxAccessGatewayBlockingStub stub =
+                    MxAccessGatewayGrpc.newBlockingStub(channel);
+            assertThrows(StatusRuntimeException.class, () ->
+                    stub.openSession(OpenSessionRequest.newBuilder()
+                            .setClientSessionName("tls-strict-test")
+                            .build()));
+        } finally {
+            channel.shutdownNow();
+        }
+    }
+
+    /** Minimal gateway stub that succeeds any OpenSession call. */
+    private static final class MinimalGatewayService
+            extends MxAccessGatewayGrpc.MxAccessGatewayImplBase {
+        @Override
+        public void openSession(
+                OpenSessionRequest request,
+                StreamObserver<OpenSessionReply> responseObserver) {
+            responseObserver.onNext(OpenSessionReply.newBuilder()
+                    .setSessionId("tls-test-session")
+                    .setProtocolStatus(ProtocolStatus.newBuilder()
+                            .setCode(ProtocolStatusCode.PROTOCOL_STATUS_CODE_OK)
+                            .build())
+                    .build());
+            responseObserver.onCompleted();
+        }
+    }
+}
@@ -112,6 +112,28 @@ Support:
 - TLS channel with default roots,
 - custom root certificate file.

+### Trust posture (trust-on-first-use)
+
+The gateway can serve a self-signed certificate it generates itself (it has no
+PKI). grpc-python exposes no per-channel skip-verify hook, so the client cannot
+"accept any certificate" the way the other clients do. Instead, when the channel
+is not plaintext and neither `ca_file` nor `require_certificate_validation` is
+set, the TLS default is **trust-on-first-use**: the client fetches the server's
+presented certificate once via `ssl.get_server_certificate` (an unverified
+probe), pins it as the channel's only trust root, and — because the generated
+certificate always carries a `localhost` SAN — defaults
+`grpc.ssl_target_name_override` to `localhost` when no `server_name_override` was
+supplied (tolerating dial-by-IP or a hostname mismatch). A failed probe is
+surfaced as a transport error naming the endpoint.
+
+To verify the gateway instead:
+
+- set `ca_file` to verify against a specific CA, or
+- set `require_certificate_validation=True` to verify against the system trust
+  roots.
+
+Both bypass the TOFU path.
+
 ## Streaming

 Expose `stream_events` as an async iterator. Canceling the task should cancel
@@ -230,6 +230,17 @@ The client supports plaintext channels for local development, TLS with system
 roots, TLS with a custom `ca_file`, and an optional test server name override.
 API keys are redacted from option repr output and CLI error output.

+The gateway can auto-generate its own self-signed certificate (it has no PKI).
+grpc-python has no per-channel skip-verify, so the lenient TLS default is
+**trust-on-first-use**: with no `ca_file` and `require_certificate_validation`
+left `False`, the client fetches the gateway's presented certificate once
+(unverified) and pins it for the channel, defaulting the SNI/target-name override
+to `localhost` (the generated certificate always carries a `localhost` SAN) when
+none was supplied. To verify instead, pass `ca_file` to verify against a specific
+CA, or set `require_certificate_validation=True` to verify against the system
+trust roots. See
+[Gateway Configuration](../../docs/GatewayConfiguration.md#automatic-self-signed-certificate).
+
 ## CLI

 The CLI emits deterministic JSON for automation:
@@ -2,6 +2,7 @@

 from __future__ import annotations

+import ssl
 from collections.abc import Sequence
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -9,6 +10,7 @@ from pathlib import Path
 import grpc

 from .auth import REDACTED, ApiKey
+from .errors import MxGatewayTransportError


@dataclass(frozen=True)
@@ -19,6 +21,7 @@ class ClientOptions:
    api_key: str | ApiKey | None = None
    plaintext: bool = False
    ca_file: str | None = None
+    require_certificate_validation: bool = False
    server_name_override: str | None = None
    call_timeout: float | None = 30.0
    stream_timeout: float | None = None
@@ -45,6 +48,7 @@ class ClientOptions:
            f"{type(self).__name__}(endpoint={self.endpoint!r}, "
            f"api_key={api_key!r}, plaintext={self.plaintext!r}, "
            f"ca_file={self.ca_file!r}, "
+            f"require_certificate_validation={self.require_certificate_validation!r}, "
            f"server_name_override={self.server_name_override!r}, "
            f"call_timeout={self.call_timeout!r}, "
            f"stream_timeout={self.stream_timeout!r}, "
@@ -69,8 +73,34 @@ class BrowseChildrenOptions:
    historized_only: bool = False


+def _split_authority(endpoint: str) -> tuple[str, int]:
+    """Split a gRPC target (optionally scheme-prefixed) into (host, port).
+
+    Handles bracketed IPv6 literals (e.g. ``[::1]:5120`` or bare ``[::1]``),
+    returning the host without brackets so it is safe to pass to
+    ``ssl.get_server_certificate``.
+    """
+    target = endpoint.split("://", 1)[-1]
+    if target.startswith("["):
+        # Bracketed IPv6: "[::1]:5120" or "[::1]"
+        bracket_end = target.find("]")
+        host = target[1:bracket_end]  # strip surrounding brackets
+        remainder = target[bracket_end + 1 :]  # ":5120" or ""
+        port_str = remainder.lstrip(":")
+        return (host, int(port_str) if port_str else 443)
+    host, _, port = target.rpartition(":")
+    return (host or "localhost", int(port) if port else 443)
+
+
 def create_channel(options: ClientOptions) -> grpc.aio.Channel:
-    """Create a plaintext or TLS `grpc.aio` channel from client options."""
+    """Create a plaintext or TLS `grpc.aio` channel from client options.
+
+    The TLS default is lenient: grpc-python has no per-channel skip-verify, so
+    the server's presented certificate is fetched once (unverified) and pinned
+    as the channel's only trust root (trust-on-first-use). Set
+    `require_certificate_validation=True` to force system-trust verification, or
+    pass `ca_file` to verify against a specific CA — both bypass the TOFU path.
+    """

    channel_options: list[tuple[str, str | int]] = [
        ("grpc.max_receive_message_length", options.max_grpc_message_bytes),
@@ -82,11 +112,28 @@ def create_channel(options: ClientOptions) -> grpc.aio.Channel:
    if options.plaintext:
        return grpc.aio.insecure_channel(options.endpoint, options=channel_options)

-    root_certificates = None
    if options.ca_file:
        root_certificates = Path(options.ca_file).read_bytes()
+        credentials = grpc.ssl_channel_credentials(root_certificates=root_certificates)
+    elif options.require_certificate_validation:
+        credentials = grpc.ssl_channel_credentials()
+    else:
+        # Lenient default: grpc-python has no per-channel skip-verify, so fetch the
+        # server's certificate (unverified) and pin it for this channel (TOFU).
+        host, port = _split_authority(options.endpoint)
+        try:
+            presented = ssl.get_server_certificate((host, port))
+        except OSError as error:
+            raise MxGatewayTransportError(
+                f"failed to fetch TLS certificate from {options.endpoint}: {error}"
+            ) from error
+        credentials = grpc.ssl_channel_credentials(root_certificates=presented.encode("ascii"))
+        # The gateway self-signed cert always carries a "localhost" SAN, so default
+        # the SNI/target-name override to it when none was supplied, tolerating
+        # dial-by-IP or hostname mismatch.
+        if not options.server_name_override:
+            channel_options.append(("grpc.ssl_target_name_override", "localhost"))

-    credentials = grpc.ssl_channel_credentials(root_certificates=root_certificates)
    return grpc.aio.secure_channel(
        options.endpoint,
        credentials,
@@ -72,27 +72,83 @@ def test_create_channel_uses_plaintext_channel(monkeypatch: pytest.MonkeyPatch)
    ]


-def test_create_channel_uses_tls_channel(monkeypatch: pytest.MonkeyPatch) -> None:
-    calls: list[tuple[str, object, object]] = []
+def test_create_channel_uses_tls_channel_tofu_default(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Default TLS (no ca_file, no require_certificate_validation) uses TOFU:
+    fetches the server cert unverified, pins it as root_certificates, and adds
+    grpc.ssl_target_name_override = "localhost" automatically.
+    """
+    _DUMMY_PEM = "-----BEGIN CERTIFICATE-----\nZmFrZQ==\n-----END CERTIFICATE-----\n"
+    get_cert_calls: list[tuple[str, int]] = []

-    def fake_credentials(*, root_certificates: object) -> str:
-        assert root_certificates is None
+    def fake_get_server_certificate(addr: tuple[str, int]) -> str:
+        get_cert_calls.append(addr)
+        return _DUMMY_PEM
+
+    cred_calls: list[object] = []
+
+    def fake_credentials(*, root_certificates: object = None) -> str:
+        cred_calls.append(root_certificates)
        return "creds"

+    channel_calls: list[tuple[str, object, object]] = []
+
    def fake_secure_channel(endpoint: str, credentials: object, *, options: object) -> str:
-        calls.append((endpoint, credentials, options))
+        channel_calls.append((endpoint, credentials, options))
        return "tls-channel"

-    monkeypatch.setattr(
-        options_module.grpc,
-        "ssl_channel_credentials",
-        fake_credentials,
+    monkeypatch.setattr(options_module.ssl, "get_server_certificate", fake_get_server_certificate)
+    monkeypatch.setattr(options_module.grpc, "ssl_channel_credentials", fake_credentials)
+    monkeypatch.setattr(options_module.grpc.aio, "secure_channel", fake_secure_channel)
+
+    channel = create_channel(
+        ClientOptions(endpoint="gateway.example:5001"),
    )
+
+    assert channel == "tls-channel"
+    # TOFU: should have fetched the cert from the server (host, port)
+    assert get_cert_calls == [("gateway.example", 5001)]
+    # Pinned the fetched PEM bytes as root_certificates
+    assert cred_calls == [_DUMMY_PEM.encode("ascii")]
+    # Auto-injected localhost override (no server_name_override supplied)
+    assert channel_calls == [
+        (
+            "gateway.example:5001",
+            "creds",
+            [
+                ("grpc.max_receive_message_length", 16 * 1024 * 1024),
+                ("grpc.max_send_message_length", 16 * 1024 * 1024),
+                ("grpc.ssl_target_name_override", "localhost"),
+            ],
+        ),
+    ]
+
+
+def test_create_channel_uses_tls_channel_tofu_respects_server_name_override(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """When server_name_override is set, TOFU still runs but does NOT add the
+    auto-localhost override (the explicit override is already in channel_options).
+    """
+    _DUMMY_PEM = "-----BEGIN CERTIFICATE-----\nZmFrZQ==\n-----END CERTIFICATE-----\n"
    monkeypatch.setattr(
-        options_module.grpc.aio,
-        "secure_channel",
-        fake_secure_channel,
+        options_module.ssl,
+        "get_server_certificate",
+        lambda addr: _DUMMY_PEM,
    )
+    cred_calls: list[object] = []
+
+    def fake_credentials(*, root_certificates: object = None) -> str:
+        cred_calls.append(root_certificates)
+        return "creds"
+
+    channel_calls: list[tuple[str, object, object]] = []
+
+    def fake_secure_channel(endpoint: str, credentials: object, *, options: object) -> str:
+        channel_calls.append((endpoint, credentials, options))
+        return "tls-channel"
+
+    monkeypatch.setattr(options_module.grpc, "ssl_channel_credentials", fake_credentials)
+    monkeypatch.setattr(options_module.grpc.aio, "secure_channel", fake_secure_channel)

    channel = create_channel(
        ClientOptions(
@@ -102,14 +158,121 @@ def test_create_channel_uses_tls_channel(monkeypatch: pytest.MonkeyPatch) -> Non
    )

    assert channel == "tls-channel"
-    assert calls == [
-            (
-                "gateway.example:5001",
-                "creds",
-                [
-                    ("grpc.max_receive_message_length", 16 * 1024 * 1024),
-                    ("grpc.max_send_message_length", 16 * 1024 * 1024),
-                    ("grpc.ssl_target_name_override", "gateway.test"),
-                ],
-            ),
-        ]
+    assert cred_calls == [_DUMMY_PEM.encode("ascii")]
+    assert channel_calls == [
+        (
+            "gateway.example:5001",
+            "creds",
+            [
+                ("grpc.max_receive_message_length", 16 * 1024 * 1024),
+                ("grpc.max_send_message_length", 16 * 1024 * 1024),
+                # Explicit override from ClientOptions — not the auto-localhost one
+                ("grpc.ssl_target_name_override", "gateway.test"),
+            ],
+        ),
+    ]
+
+
+def test_create_channel_uses_tls_channel_require_cert_validation(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """require_certificate_validation=True uses system trust (no TOFU, no root_certificates)."""
+    get_cert_called = False
+
+    def fake_get_server_certificate(addr: object) -> str:  # pragma: no cover
+        nonlocal get_cert_called
+        get_cert_called = True
+        return "SHOULD_NOT_BE_CALLED"
+
+    cred_calls: list[object] = []
+
+    def fake_credentials(**kwargs: object) -> str:
+        cred_calls.append(kwargs)
+        return "creds"
+
+    channel_calls: list[tuple[str, object, object]] = []
+
+    def fake_secure_channel(endpoint: str, credentials: object, *, options: object) -> str:
+        channel_calls.append((endpoint, credentials, options))
+        return "tls-channel"
+
+    monkeypatch.setattr(options_module.ssl, "get_server_certificate", fake_get_server_certificate)
+    monkeypatch.setattr(options_module.grpc, "ssl_channel_credentials", fake_credentials)
+    monkeypatch.setattr(options_module.grpc.aio, "secure_channel", fake_secure_channel)
+
+    channel = create_channel(
+        ClientOptions(
+            endpoint="gateway.example:5001",
+            require_certificate_validation=True,
+        ),
+    )
+
+    assert channel == "tls-channel"
+    # Must NOT call TOFU prefetch
+    assert not get_cert_called
+    # ssl_channel_credentials() called with NO keyword args (system trust)
+    assert cred_calls == [{}]
+    assert channel_calls == [
+        (
+            "gateway.example:5001",
+            "creds",
+            [
+                ("grpc.max_receive_message_length", 16 * 1024 * 1024),
+                ("grpc.max_send_message_length", 16 * 1024 * 1024),
+            ],
+        ),
+    ]
+
+
+def test_create_channel_uses_tls_channel_ca_file(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: pytest.TempPathFactory,
+) -> None:
+    """ca_file path: reads the PEM file, passes bytes as root_certificates, skips TOFU."""
+    ca_pem = b"-----BEGIN CERTIFICATE-----\nY2FkYXRh\n-----END CERTIFICATE-----\n"
+    ca_file = tmp_path / "ca.pem"
+    ca_file.write_bytes(ca_pem)
+
+    get_cert_called = False
+
+    def fake_get_server_certificate(addr: object) -> str:  # pragma: no cover
+        nonlocal get_cert_called
+        get_cert_called = True
+        return "SHOULD_NOT_BE_CALLED"
+
+    cred_calls: list[object] = []
+
+    def fake_credentials(*, root_certificates: object = None) -> str:
+        cred_calls.append(root_certificates)
+        return "creds"
+
+    channel_calls: list[tuple[str, object, object]] = []
+
+    def fake_secure_channel(endpoint: str, credentials: object, *, options: object) -> str:
+        channel_calls.append((endpoint, credentials, options))
+        return "tls-channel"
+
+    monkeypatch.setattr(options_module.ssl, "get_server_certificate", fake_get_server_certificate)
+    monkeypatch.setattr(options_module.grpc, "ssl_channel_credentials", fake_credentials)
+    monkeypatch.setattr(options_module.grpc.aio, "secure_channel", fake_secure_channel)
+
+    channel = create_channel(
+        ClientOptions(
+            endpoint="gateway.example:5001",
+            ca_file=str(ca_file),
+        ),
+    )
+
+    assert channel == "tls-channel"
+    assert not get_cert_called
+    assert cred_calls == [ca_pem]
+    assert channel_calls == [
+        (
+            "gateway.example:5001",
+            "creds",
+            [
+                ("grpc.max_receive_message_length", 16 * 1024 * 1024),
+                ("grpc.max_send_message_length", 16 * 1024 * 1024),
+            ],
+        ),
+    ]
@@ -0,0 +1,165 @@
+"""TLS behaviour tests for ``create_channel``.
+
+These spin up a real loopback ``grpc.aio`` server with a freshly generated
+self-signed certificate (carrying a ``localhost`` SAN, mirroring the gateway's
+auto-generated cert) and assert the lenient TOFU default lets a client connect
+without any CA configured.
+
+Marked ``tls`` and skipped unless ``MXGATEWAY_RUN_TLS_TESTS=1`` because loopback
+TLS handshakes can be timing-flaky on shared CI runners. This mirrors how the
+suite gates anything that depends on real sockets rather than fakes.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import socket
+import ssl
+import subprocess
+import tempfile
+from collections.abc import AsyncIterator
+from pathlib import Path
+
+import grpc
+import pytest
+import pytest_asyncio
+
+from zb_mom_ww_mxgateway import ClientOptions
+from zb_mom_ww_mxgateway.errors import MxGatewayTransportError
+from zb_mom_ww_mxgateway.generated import mxaccess_gateway_pb2 as pb
+from zb_mom_ww_mxgateway.generated import mxaccess_gateway_pb2_grpc as pb_grpc
+from zb_mom_ww_mxgateway.options import create_channel
+
+pytestmark = pytest.mark.tls
+
+_RUN_TLS_TESTS = os.environ.get("MXGATEWAY_RUN_TLS_TESTS") == "1"
+_OPENSSL = shutil.which("openssl")
+
+requires_tls = pytest.mark.skipif(
+    not _RUN_TLS_TESTS,
+    reason="set MXGATEWAY_RUN_TLS_TESTS=1 to run loopback TLS tests",
+)
+requires_openssl = pytest.mark.skipif(
+    _OPENSSL is None,
+    reason="openssl CLI is required to generate a self-signed test certificate",
+)
+
+
+def _generate_self_signed_cert(directory: Path) -> tuple[Path, Path]:
+    """Generate a self-signed cert/key pair with a ``localhost`` SAN."""
+    key_path = directory / "server.key"
+    cert_path = directory / "server.crt"
+    subprocess.run(
+        [
+            str(_OPENSSL),
+            "req",
+            "-x509",
+            "-newkey",
+            "rsa:2048",
+            "-nodes",
+            "-keyout",
+            str(key_path),
+            "-out",
+            str(cert_path),
+            "-days",
+            "1",
+            "-subj",
+            "/CN=mxgateway-test",
+            "-addext",
+            "subjectAltName=DNS:localhost,IP:127.0.0.1",
+        ],
+        check=True,
+        capture_output=True,
+    )
+    return cert_path, key_path
+
+
+def _free_port() -> int:
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+        sock.bind(("127.0.0.1", 0))
+        return int(sock.getsockname()[1])
+
+
+class _StaticGatewayServicer(pb_grpc.MxAccessGatewayServicer):
+    """Minimal servicer answering ``OpenSession`` with a fixed session id."""
+
+    async def OpenSession(  # noqa: N802 - generated gRPC method name
+        self, request: pb.OpenSessionRequest, context: object
+    ) -> pb.OpenSessionReply:
+        return pb.OpenSessionReply(session_id="tls-session-1")
+
+
+@pytest_asyncio.fixture
+async def tls_server() -> AsyncIterator[int]:
+    with tempfile.TemporaryDirectory() as tmp:
+        cert_path, key_path = _generate_self_signed_cert(Path(tmp))
+        credentials = grpc.ssl_server_credentials(
+            [(key_path.read_bytes(), cert_path.read_bytes())]
+        )
+        server = grpc.aio.server()
+        pb_grpc.add_MxAccessGatewayServicer_to_server(_StaticGatewayServicer(), server)
+        port = _free_port()
+        server.add_secure_port(f"127.0.0.1:{port}", credentials)
+        await server.start()
+        try:
+            yield port
+        finally:
+            await server.stop(grace=None)
+
+
+@requires_tls
+@requires_openssl
+@pytest.mark.asyncio
+async def test_default_tls_connects_via_tofu(tls_server: int) -> None:
+    """Default TLS options (no CA) connect by pinning the presented cert."""
+    options = ClientOptions(
+        endpoint=f"127.0.0.1:{tls_server}",
+        api_key="mxgw_test_secret",
+    )
+    channel = create_channel(options)
+    try:
+        stub = pb_grpc.MxAccessGatewayStub(channel)
+        reply = await stub.OpenSession(pb.OpenSessionRequest(), timeout=10)
+        assert reply.session_id == "tls-session-1"
+    finally:
+        await channel.close()
+
+
+def test_split_authority_parses_host_and_port() -> None:
+    from zb_mom_ww_mxgateway.options import _split_authority
+
+    assert _split_authority("https://10.0.0.5:5120") == ("10.0.0.5", 5120)
+    assert _split_authority("localhost:5120") == ("localhost", 5120)
+    assert _split_authority(":5120") == ("localhost", 5120)
+
+
+def test_split_authority_strips_ipv6_brackets() -> None:
+    from zb_mom_ww_mxgateway.options import _split_authority
+
+    # Bracketed IPv6 with port — brackets must be removed for ssl.get_server_certificate
+    assert _split_authority("[::1]:5120") == ("::1", 5120)
+    # Bare bracketed IPv6 (no port) — default port 443
+    assert _split_authority("[::1]") == ("::1", 443)
+    # Scheme-prefixed bracketed IPv6
+    assert _split_authority("grpc://[::1]:5120") == ("::1", 5120)
+
+
+def test_tofu_connect_failure_raises_transport_error() -> None:
+    """A failed cert pre-fetch surfaces the client's transport error type."""
+    options = ClientOptions(endpoint=f"127.0.0.1:{_free_port()}")
+    with pytest.raises(MxGatewayTransportError) as excinfo:
+        create_channel(options)
+    assert options.endpoint in str(excinfo.value)
+
+
+def test_require_certificate_validation_uses_system_trust() -> None:
+    """``require_certificate_validation`` must not attempt a TOFU pre-fetch."""
+    # Pointing at a closed port: with system-trust the channel is created lazily
+    # (no eager pre-fetch), so create_channel must succeed without connecting.
+    options = ClientOptions(
+        endpoint=f"127.0.0.1:{_free_port()}",
+        require_certificate_validation=True,
+    )
+    channel = create_channel(options)
+    assert isinstance(channel, grpc.aio.Channel)
@@ -76,6 +76,19 @@ types.
 cargo run -p mxgw-cli -- smoke --endpoint https://mxgateway.example.local:5001 --tls --ca-file C:\certs\mxgateway-ca.pem --server-name-override mxgateway.example.local --api-key-env MXGATEWAY_API_KEY --item TestChildObject.TestInt --json
 ```

+### TLS trust (pin-only)
+
+The gateway can auto-generate its own self-signed certificate (it has no PKI).
+Unlike the other clients, the Rust client is **not** lenient: tonic 0.13.1
+exposes no public hook to inject a custom certificate verifier, so TLS over Rust
+is pin-only. A TLS connection requires either `--ca-file` /
+`ClientOptions::with_ca_file(...)` to pin a CA (export the gateway's self-signed
+certificate and pin it), or `--require-certificate-validation` /
+`with_require_certificate_validation(true)` to verify against the system trust
+roots. TLS with neither set fails `connect` with a clear, actionable error rather
+than accepting the certificate. See
+[Gateway Configuration](../../docs/GatewayConfiguration.md#automatic-self-signed-certificate).
+
 ## Library Surface

 `ClientOptions` configures endpoint, API key, plaintext or TLS transport,
@@ -189,6 +189,25 @@ Support:
 - custom CA file,
 - domain override.

+### Trust posture (pin-only)
+
+The gateway can serve a self-signed certificate it generates itself (it has no
+PKI). Rust is the **exception** to the lenient-by-default posture the other
+clients use: tonic 0.13.1 exposes no public hook to inject a custom certificate
+verifier, so the Rust client cannot accept an arbitrary certificate. TLS over the
+Rust client is therefore **pin-only** — it requires either:
+
+- `ClientOptions::with_ca_file(...)` to pin a CA (the supported path for the
+  gateway's self-signed certificate; export the certificate and pin it), or
+- `ClientOptions::with_require_certificate_validation(true)` to verify against the
+  system trust roots.
+
+With TLS enabled (`with_plaintext(false)`), no pinned CA, and certificate
+validation not required, `GatewayClient::connect` rejects the connection with a
+clear, actionable error pointing at `with_ca_file` /
+`require_certificate_validation` rather than silently accepting the certificate.
+The CLI exposes `--ca-file` and `--require-certificate-validation`.
+
 ## Streaming

 Expose event streams as a `Stream<Item = Result<MxEvent, Error>>`. Dropping the
@@ -426,6 +426,11 @@ struct ConnectionArgs {
    ca_file: Option<PathBuf>,
    #[arg(long)]
    server_name_override: Option<String>,
+    /// Verify the server certificate against the system trust roots even
+    /// without a pinned CA. The Rust client's default is to require a CA
+    /// file (see `--ca-file`); set this flag to use system roots instead.
+    #[arg(long)]
+    require_certificate_validation: bool,
    #[arg(long, default_value_t = 10)]
    connect_timeout_seconds: u64,
    #[arg(long, default_value_t = 30)]
@@ -453,6 +458,9 @@ impl ConnectionArgs {
        if let Some(server_name_override) = &self.server_name_override {
            options = options.with_server_name_override(server_name_override);
        }
+        if self.require_certificate_validation {
+            options = options.with_require_certificate_validation(true);
+        }

        options
    }
@@ -6,10 +6,8 @@
 //! code should prefer [`GatewayClient::open_session`] and the [`Session`]
 //! handle it returns, rather than the `*_raw` methods.

-use std::fs;
-
 use tonic::codegen::InterceptedService;
-use tonic::transport::{Certificate, Channel, ClientTlsConfig};
+use tonic::transport::Channel;
 use tonic::Request;

 use crate::auth::AuthInterceptor;
@@ -21,7 +19,7 @@ use crate::generated::mxaccess_gateway::v1::{
    OpenSessionReply, OpenSessionRequest, QueryActiveAlarmsRequest, StreamAlarmsRequest,
    StreamEventsRequest,
 };
-use crate::options::ClientOptions;
+use crate::options::{build_tls_config, ClientOptions};
 use crate::session::Session;

 /// Generated gateway client wrapped in the auth interceptor that
@@ -78,18 +76,7 @@ impl GatewayClient {
            })?;
        endpoint = endpoint.connect_timeout(options.connect_timeout());

-        if !options.plaintext() {
-            let mut tls = ClientTlsConfig::new();
-            if let Some(server_name) = options.server_name_override() {
-                tls = tls.domain_name(server_name.to_owned());
-            }
-            if let Some(ca_file) = options.ca_file() {
-                let certificate = fs::read(ca_file).map_err(|source| Error::InvalidEndpoint {
-                    endpoint: options.endpoint().to_owned(),
-                    detail: format!("failed to read CA file {}: {source}", ca_file.display()),
-                })?;
-                tls = tls.ca_certificate(Certificate::from_pem(certificate));
-            }
+        if let Some(tls) = build_tls_config(&options)? {
            endpoint = endpoint.tls_config(tls)?;
        }

@@ -6,13 +6,12 @@
 //! re-exported through [`crate::generated::galaxy_repository::v1`].

 use std::collections::HashSet;
-use std::fs;
 use std::sync::Arc;

 use prost_types::Timestamp;
 use tokio::sync::Mutex as AsyncMutex;
 use tonic::codegen::InterceptedService;
-use tonic::transport::{Certificate, Channel, ClientTlsConfig};
+use tonic::transport::Channel;
 use tonic::Request;

 use crate::auth::AuthInterceptor;
@@ -23,7 +22,7 @@ use crate::generated::galaxy_repository::v1::{
    DiscoverHierarchyRequest, GalaxyObject, GetLastDeployTimeRequest, TestConnectionRequest,
    WatchDeployEventsRequest,
 };
-use crate::options::ClientOptions;
+use crate::options::{build_tls_config, ClientOptions};

 const DISCOVER_HIERARCHY_PAGE_SIZE: i32 = 5000;
 const BROWSE_CHILDREN_PAGE_SIZE: i32 = 500;
@@ -183,18 +182,7 @@ impl GalaxyClient {
            })?;
        endpoint = endpoint.connect_timeout(options.connect_timeout());

-        if !options.plaintext() {
-            let mut tls = ClientTlsConfig::new();
-            if let Some(server_name) = options.server_name_override() {
-                tls = tls.domain_name(server_name.to_owned());
-            }
-            if let Some(ca_file) = options.ca_file() {
-                let certificate = fs::read(ca_file).map_err(|source| Error::InvalidEndpoint {
-                    endpoint: options.endpoint().to_owned(),
-                    detail: format!("failed to read CA file {}: {source}", ca_file.display()),
-                })?;
-                tls = tls.ca_certificate(Certificate::from_pem(certificate));
-            }
+        if let Some(tls) = build_tls_config(&options)? {
            endpoint = endpoint.tls_config(tls)?;
        }

@@ -3,10 +3,14 @@
 //! chain of `with_*` setters; the `Debug` impl redacts the API key.

 use std::fmt;
+use std::fs;
 use std::path::PathBuf;
 use std::time::Duration;

+use tonic::transport::{Certificate, ClientTlsConfig};
+
 use crate::auth::ApiKey;
+use crate::error::Error;

 const DEFAULT_MAX_GRPC_MESSAGE_BYTES: usize = 16 * 1024 * 1024;

@@ -22,6 +26,7 @@ pub struct ClientOptions {
    api_key: Option<ApiKey>,
    plaintext: bool,
    ca_file: Option<PathBuf>,
+    require_certificate_validation: bool,
    server_name_override: Option<String>,
    connect_timeout: Duration,
    call_timeout: Duration,
@@ -38,6 +43,7 @@ impl ClientOptions {
            api_key: None,
            plaintext: true,
            ca_file: None,
+            require_certificate_validation: false,
            server_name_override: None,
            connect_timeout: Duration::from_secs(10),
            call_timeout: Duration::from_secs(30),
@@ -67,6 +73,22 @@ impl ClientOptions {
        self
    }

+    /// Require TLS certificate verification even without a pinned CA. Default
+    /// false: the gateway's self-signed certificate is accepted (internal-tool
+    /// posture). Setting a CA file always verifies.
+    ///
+    /// Note for Rust: tonic 0.13's `ClientTlsConfig` exposes no hook for a
+    /// custom rustls verifier, so the Rust client cannot accept an arbitrary
+    /// self-signed certificate the way the other clients do. With the default
+    /// (false) and no pinned CA, [`crate::client::GatewayClient::connect`]
+    /// rejects the TLS connection and asks for a CA file. Either pin a CA via
+    /// [`ClientOptions::with_ca_file`] (the supported lenient path on Rust) or
+    /// set this `true` to verify against the system trust roots.
+    pub fn with_require_certificate_validation(mut self, require: bool) -> Self {
+        self.require_certificate_validation = require;
+        self
+    }
+
    /// Override the SNI/server name used during the TLS handshake. Useful
    /// when the dial-target host name does not match the certificate.
    pub fn with_server_name_override(mut self, server_name_override: impl Into<String>) -> Self {
@@ -121,6 +143,12 @@ impl ClientOptions {
        self.ca_file.as_ref()
    }

+    /// Whether TLS certificate verification is required even without a pinned
+    /// CA. See [`ClientOptions::with_require_certificate_validation`].
+    pub fn require_certificate_validation(&self) -> bool {
+        self.require_certificate_validation
+    }
+
    /// Optional SNI / server-name override for TLS handshakes.
    pub fn server_name_override(&self) -> Option<&str> {
        self.server_name_override.as_deref()
@@ -147,6 +175,68 @@ impl ClientOptions {
    }
 }

+/// Build the [`ClientTlsConfig`] for a non-plaintext connection described by
+/// `options`, applying the lenient-default guard that is the **Rust
+/// pin-only exception**.
+///
+/// Returns `Ok(None)` when `options.plaintext()` is `true` (no TLS needed).
+/// Returns `Ok(Some(tls))` when a valid TLS config can be assembled.
+/// Returns `Err(Error::InvalidEndpoint)` when TLS is requested but no pinned
+/// CA was provided and `require_certificate_validation` is `false`.
+///
+/// # Why this guard exists
+///
+/// `tonic` 0.13's `ClientTlsConfig` builds its rustls verifier inside a
+/// crate-private connector and exposes no hook for a custom
+/// `ServerCertVerifier`. The Rust client therefore cannot accept an arbitrary
+/// self-signed certificate the way the other language clients do. Rather than
+/// silently falling back to system-root verification (which always fails
+/// against a self-signed gateway certificate), we reject the configuration
+/// early with an actionable error.
+pub(crate) fn build_tls_config(options: &ClientOptions) -> Result<Option<ClientTlsConfig>, Error> {
+    if options.plaintext() {
+        return Ok(None);
+    }
+
+    let mut tls = ClientTlsConfig::new();
+    if let Some(server_name) = options.server_name_override() {
+        tls = tls.domain_name(server_name.to_owned());
+    }
+    if let Some(ca_file) = options.ca_file() {
+        let certificate = fs::read(ca_file).map_err(|source| Error::InvalidEndpoint {
+            endpoint: options.endpoint().to_owned(),
+            detail: format!("failed to read CA file {}: {source}", ca_file.display()),
+        })?;
+        tls = tls.ca_certificate(Certificate::from_pem(certificate));
+    } else if !options.require_certificate_validation() {
+        // Lenient-default fallback (Rust pin-only exception): tonic
+        // 0.13's `ClientTlsConfig` builds its rustls verifier inside a
+        // crate-private connector and exposes no hook for a custom
+        // `ServerCertVerifier`, so — unlike the other clients — the
+        // Rust client cannot accept an arbitrary self-signed cert. Pin
+        // the gateway's CA instead, or opt into strict verification
+        // against the system trust roots. We reject here rather than
+        // silently verifying against system roots (which would fail a
+        // self-signed gateway with a confusing handshake error).
+        //
+        // Note: a server-name override affects SNI (the hostname sent
+        // in the TLS ClientHello) but does NOT pin trust. Overriding
+        // the server name alone does not bypass certificate validation.
+        return Err(Error::InvalidEndpoint {
+            endpoint: options.endpoint().to_owned(),
+            detail: "TLS requested without a pinned CA. The Rust client cannot accept an \
+                     arbitrary self-signed certificate (tonic 0.13 exposes no custom \
+                     rustls verifier). Pin the gateway certificate with \
+                     ClientOptions::with_ca_file, or call \
+                     ClientOptions::with_require_certificate_validation(true) to verify \
+                     against the system trust roots. Note: a server-name override \
+                     affects SNI but does not pin trust."
+                .to_owned(),
+        });
+    }
+    Ok(Some(tls))
+}
+
 impl Default for ClientOptions {
    fn default() -> Self {
        Self::new("http://127.0.0.1:5000")
@@ -161,6 +251,10 @@ impl fmt::Debug for ClientOptions {
            .field("api_key", &self.api_key.as_ref().map(|_| "<redacted>"))
            .field("plaintext", &self.plaintext)
            .field("ca_file", &self.ca_file)
+            .field(
+                "require_certificate_validation",
+                &self.require_certificate_validation,
+            )
            .field("server_name_override", &self.server_name_override)
            .field("connect_timeout", &self.connect_timeout)
            .field("call_timeout", &self.call_timeout)
@@ -0,0 +1,137 @@
+//! TLS posture coverage for the Rust client.
+//!
+//! tonic 0.13.1's `ClientTlsConfig` exposes no hook for a custom rustls
+//! `ServerCertVerifier` (the verifier is built internally inside the
+//! crate-private `TlsConnector`), so the Rust client cannot implement the
+//! "accept any server certificate" lenient default the other clients use.
+//! Rust is therefore the documented **pin-only exception**: TLS without a
+//! pinned CA is rejected up front with a clear, actionable error, and
+//! supplying a CA file is the supported path. These tests pin that contract.
+
+use std::time::Duration;
+
+use zb_mom_ww_mxgateway_client::{ClientOptions, Error, GalaxyClient, GatewayClient};
+
+/// Drive `connect` to its error without requiring `GatewayClient: Debug`
+/// (the success arm is dropped explicitly so `unwrap_err` is unnecessary).
+async fn connect_err(options: ClientOptions) -> Error {
+    match GatewayClient::connect(options).await {
+        Ok(_client) => panic!("connect unexpectedly succeeded against a dead TLS address"),
+        Err(error) => error,
+    }
+}
+
+#[tokio::test]
+async fn tls_without_ca_is_rejected_with_actionable_error_by_default() {
+    let options = ClientOptions::new("https://127.0.0.1:1")
+        .with_plaintext(false)
+        .with_connect_timeout(Duration::from_millis(200));
+
+    let error = connect_err(options).await;
+
+    let Error::InvalidEndpoint { detail, .. } = error else {
+        panic!("expected InvalidEndpoint, got {error:?}");
+    };
+    // The message must point the caller at the supported remedy (pin a CA)
+    // and name the opt-in escape hatch.
+    assert!(
+        detail.contains("ca_file") || detail.contains("CA"),
+        "error should instruct the user to pass a CA file: {detail}"
+    );
+    assert!(
+        detail.contains("require_certificate_validation"),
+        "error should mention the require_certificate_validation opt-in: {detail}"
+    );
+}
+
+#[tokio::test]
+async fn tls_with_require_certificate_validation_does_not_short_circuit() {
+    // With strict verification opted in, the no-CA guard must not fire; the
+    // connect attempt instead proceeds to the transport (and fails to reach
+    // the dead address) rather than returning the "CA required" guard error.
+    let options = ClientOptions::new("https://127.0.0.1:1")
+        .with_plaintext(false)
+        .with_require_certificate_validation(true)
+        .with_connect_timeout(Duration::from_millis(200));
+
+    let error = connect_err(options).await;
+
+    assert!(
+        !matches!(&error, Error::InvalidEndpoint { detail, .. }
+            if detail.contains("require_certificate_validation")),
+        "strict verification must bypass the no-CA guard, got {error:?}"
+    );
+}
+
+#[tokio::test]
+async fn tls_with_ca_file_is_permitted_and_proceeds_past_the_guard() {
+    // Pinning a CA is the supported TLS path: the no-CA guard must not fire.
+    // We hand it a readable PEM file; construction proceeds past the guard
+    // and only fails later at the transport (dead address / handshake).
+    let ca_path = std::env::temp_dir().join("mxgw-rust-tls-ca-fixture.pem");
+    std::fs::write(&ca_path, SELF_SIGNED_CA_PEM).unwrap();
+
+    let options = ClientOptions::new("https://127.0.0.1:1")
+        .with_plaintext(false)
+        .with_ca_file(&ca_path)
+        .with_connect_timeout(Duration::from_millis(200));
+
+    let error = connect_err(options).await;
+
+    let _ = std::fs::remove_file(&ca_path);
+
+    assert!(
+        !matches!(&error, Error::InvalidEndpoint { detail, .. }
+            if detail.contains("require_certificate_validation")),
+        "pinning a CA must bypass the no-CA guard, got {error:?}"
+    );
+}
+
+/// Drive `GalaxyClient::connect` to its error (mirrors `connect_err` above).
+async fn galaxy_connect_err(options: ClientOptions) -> Error {
+    match GalaxyClient::connect(options).await {
+        Ok(_client) => {
+            panic!("GalaxyClient::connect unexpectedly succeeded against a dead TLS address")
+        }
+        Err(error) => error,
+    }
+}
+
+#[tokio::test]
+async fn galaxy_tls_without_ca_is_rejected_with_actionable_error_by_default() {
+    // GalaxyClient::connect must apply the same TLS guard as GatewayClient —
+    // TLS without a pinned CA (and without require_certificate_validation)
+    // returns a clear, actionable InvalidEndpoint error.
+    let options = ClientOptions::new("https://127.0.0.1:1")
+        .with_plaintext(false)
+        .with_connect_timeout(Duration::from_millis(200));
+
+    let error = galaxy_connect_err(options).await;
+
+    let Error::InvalidEndpoint { detail, .. } = error else {
+        panic!("expected InvalidEndpoint, got {error:?}");
+    };
+    assert!(
+        detail.contains("ca_file") || detail.contains("CA"),
+        "error should instruct the user to pass a CA file: {detail}"
+    );
+    assert!(
+        detail.contains("require_certificate_validation"),
+        "error should mention the require_certificate_validation opt-in: {detail}"
+    );
+}
+
+/// A throwaway self-signed CA certificate (PEM). Only needs to parse as a
+/// PEM trust root so the CA-pinning path is exercised past the guard.
+const SELF_SIGNED_CA_PEM: &str = "-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+";
@@ -51,6 +51,19 @@ The shared inputs are:
 The commands in the matrix use `MXGATEWAY_API_KEY` through each CLI's
 `api-key-env` flag. They must not embed bearer tokens or raw API keys.

+### TLS variant
+
+The matrix runs over plaintext (`h2c`) by default. A TLS variant exists but stays
+a manual/opt-in run, consistent with the gate above, because it needs the gateway
+started with an HTTPS endpoint (an `https://` `MXGATEWAY_ENDPOINT`) and each CLI
+switched to its TLS flag (`--tls` / `-tls` / `--plaintext=false` /
+`plaintext=False`). The clients are lenient by default and accept the gateway's
+auto-generated self-signed certificate without extra trust setup, except the Rust
+CLI, which is pin-only and needs `--ca-file` or `--require-certificate-validation`
+(and Python uses trust-on-first-use). See
+[Gateway Configuration — Automatic self-signed certificate](./GatewayConfiguration.md#automatic-self-signed-certificate)
+and each client README for the per-client TLS flags.
+
 ## JSON Comparison

 Every command in the matrix requests JSON output. A runner can compare the
@@ -375,6 +375,42 @@ deployment-heavy box, multiply per-session SQL connections, and complicate the
 cold-start path. Wire-side laziness solves the actual pain (oversized gRPC
 replies and a heavy DOM) without disturbing the materialization model.

+## TLS Auto-Certificate and Lenient Client Trust
+
+Decision: when a Kestrel `https://` endpoint is configured without a certificate
+of its own (and no `Kestrel:Certificates:Default` is set), the gateway generates
+and persists a self-signed certificate rather than failing to start. Clients
+connecting over TLS without a pinned CA accept whatever certificate the server
+presents by default; pinning a CA restores full verification.
+
+Rationale: `mxaccessgw` is an internal tool with no PKI to issue or distribute
+certificates. The prior behavior — an `https` endpoint with no certificate
+fails at startup with Kestrel's opaque "no server certificate was specified"
+error — pushed operators toward plaintext (`h2c`), exposing the API key and
+request payloads on the wire. Auto-generating a long-lived, persisted, reused
+certificate lets TLS "just work" with zero certificate management, while the
+lenient client default means clients connect to that self-signed certificate
+without a manual trust step. Both choices are deliberate, not oversights:
+strict-by-default would force PKI work this tool does not warrant. Plaintext-only
+deployments are untouched — no certificate or key material is written for them —
+and an operator who supplies a real certificate transparently overrides the
+generated one.
+
+Two clients diverge from "accept any certificate" because their gRPC stacks lack
+a per-channel skip-verify hook:
+
+- Python uses trust-on-first-use: it fetches the server's presented certificate
+  over a separate unverified probe and pins it for the channel, and defaults the
+  SNI/target-name override to `localhost` (the generated certificate always
+  carries a `localhost` SAN).
+- Rust is pin-only: tonic exposes no public hook to inject a custom certificate
+  verifier, so TLS over Rust requires either a pinned CA or an explicit opt-in to
+  system-trust verification; otherwise connecting returns a clear, actionable
+  error.
+
+See [Gateway Configuration — Automatic self-signed certificate](./GatewayConfiguration.md#automatic-self-signed-certificate)
+and the per-client READMEs for the as-built behavior.
+
 ## Later Revisit Items

 These are explicit post-v1 revisit items, not open blockers:
@@ -229,6 +229,185 @@ behavior.
 The alarm monitor is independent of client sessions: `AcknowledgeAlarm` and
 `StreamAlarms` are session-less RPCs served by the monitor.

+## Host Endpoints and Transport Security (Kestrel)
+
+The listening endpoints are **not** part of the `MxGateway` section. The gateway
+uses the stock ASP.NET Core host (`WebApplication.CreateBuilder`) with no
+`ConfigureKestrel` call in code, so endpoints come entirely from the standard
+`Kestrel` configuration section. On the deployed hosts these values are supplied
+as NSSM environment variables (`Kestrel__Endpoints__...`), not from
+`appsettings.json`.
+
+Two named endpoints are bound:
+
+| Endpoint name | Purpose | Protocol requirement |
+|---|---|---|
+| `Http` | Public gRPC API (sessions, invoke, events, Galaxy browse) | HTTP/2 |
+| `Dashboard` | Blazor dashboard and SignalR hubs | HTTP/1.1 (HTTP/2 optional) |
+
+Both endpoints share one routing pipeline; the names only select which TCP port
+serves which traffic. The gRPC endpoint must negotiate **HTTP/2**, which drives
+the protocol settings below.
+
+### Plaintext (current deployments)
+
+Both running hosts (`10.100.0.48` and `wonder-app-vd03`) serve the gRPC port in
+**cleartext HTTP/2 (`h2c`)**. Because cleartext HTTP/2 has no ALPN to negotiate
+the protocol, the gRPC endpoint must be pinned to `Http2` with prior knowledge:
+
+```text
+Kestrel__Endpoints__Http__Url=http://0.0.0.0:5120
+Kestrel__Endpoints__Http__Protocols=Http2
+Kestrel__Endpoints__Dashboard__Url=http://0.0.0.0:5130
+```
+
+In this mode all client↔gateway traffic — including the
+`authorization: Bearer mxgw_...` API key and any `WriteSecured` / `AuthenticateUser`
+payloads — crosses the network **unencrypted**. This is acceptable only on a
+trusted/isolated network segment. Prefer TLS for anything else.
+
+### TLS
+
+To encrypt the gRPC channel, give the `Http` endpoint an `https://` URL and a
+certificate. Over TLS, ALPN negotiates HTTP/2, so the explicit `Protocols=Http2`
+pin is no longer required (the default `Http1AndHttp2` works for gRPC over TLS).
+
+`appsettings.json` form:
+
+```json
+{
+  "Kestrel": {
+    "Endpoints": {
+      "Http": {
+        "Url": "https://0.0.0.0:5120",
+        "Certificate": {
+          "Path": "C:\\ProgramData\\MxGateway\\certs\\gateway.pfx",
+          "Password": "<pfx-password>"
+        }
+      },
+      "Dashboard": {
+        "Url": "https://0.0.0.0:5130",
+        "Certificate": {
+          "Path": "C:\\ProgramData\\MxGateway\\certs\\gateway.pfx",
+          "Password": "<pfx-password>"
+        }
+      }
+    }
+  }
+}
+```
+
+Equivalent NSSM environment-variable form (how config is delivered on the hosts —
+see [server deploy mechanics in the project notes]):
+
+```text
+Kestrel__Endpoints__Http__Url=https://0.0.0.0:5120
+Kestrel__Endpoints__Http__Certificate__Path=C:\ProgramData\MxGateway\certs\gateway.pfx
+Kestrel__Endpoints__Http__Certificate__Password=<pfx-password>
+Kestrel__Endpoints__Dashboard__Url=https://0.0.0.0:5130
+Kestrel__Endpoints__Dashboard__Certificate__Path=C:\ProgramData\MxGateway\certs\gateway.pfx
+Kestrel__Endpoints__Dashboard__Certificate__Password=<pfx-password>
+```
+
+Certificate sourcing options (any standard ASP.NET Core form is accepted):
+
+| Form | Keys |
+|---|---|
+| PFX file | `Certificate:Path` (+ `Certificate:Password` if encrypted) |
+| PEM pair | `Certificate:Path` (cert) + `Certificate:KeyPath` (private key) |
+| Windows cert store | `Certificate:Subject`, `Certificate:Store` (e.g. `My`), `Certificate:Location` (`LocalMachine`), `Certificate:AllowInvalid` |
+
+The certificate's CN/SAN must cover the host name clients dial (or clients must
+set a server-name override — see below). The dashboard endpoint can keep its own
+certificate independent of the gRPC endpoint; pair this with
+`MxGateway:Dashboard:RequireHttpsCookie` (`true`) for production HTTPS.
+
+### Automatic self-signed certificate
+
+`mxaccessgw` is an internal tool with no PKI to issue certificates, so requiring
+an operator to supply one before TLS works pushed deployments toward plaintext.
+To avoid that, the gateway fills in a self-signed certificate when an HTTPS
+endpoint is configured without one.
+
+**Trigger.** At startup the gateway inspects `Kestrel:Endpoints:*`. If any
+endpoint has an `https://` URL and no `Certificate` subsection of its own, and no
+`Kestrel:Certificates:Default` is set, the gateway generates (or loads) a
+persisted self-signed certificate and wires it in as the HTTPS *default* via
+`ConfigureHttpsDefaults`. All-plaintext deployments are untouched: when no HTTPS
+endpoint is configured, no certificate or key material is generated or written.
+
+**Generated certificate.** ECDSA P-256, `serverAuth` EKU, validity ≈
+`ValidityYears` (default 10 years, with one day of clock-skew slack before
+`notBefore`). SANs cover `localhost`, the machine name (and its FQDN when
+resolvable), each entry in `AdditionalDnsNames`, and the loopback addresses
+`127.0.0.1` and `::1`.
+
+**`MxGateway:Tls:*` options.** All optional; the zero-config path needs none of
+them.
+
+| Option | Default | Purpose |
+|---|---|---|
+| `Tls:SelfSignedCertPath` | `C:\ProgramData\MxGateway\certs\gateway-selfsigned.pfx` | Where the generated certificate is persisted |
+| `Tls:ValidityYears` | `10` | Lifetime of the generated certificate (validated 1–100) |
+| `Tls:AdditionalDnsNames` | `[]` | Extra DNS SANs (e.g. a load-balancer name) |
+| `Tls:RegenerateIfExpired` | `true` | Replace an expired persisted certificate instead of failing |
+
+`ValidityYears` is validated by `GatewayOptionsValidator` (range 1–100); the
+"HTTPS endpoint configured but no certificate available" fail-fast lives in the
+bootstrap/provider, because the validator only sees the `MxGateway` section, not
+`Kestrel:Endpoints`.
+
+**Persistence.** The PFX is written with an **empty** export password — a random
+in-memory password could not be reused across restarts, which the
+persist-and-reuse model requires. The private key is instead protected at rest by
+filesystem permissions: a restrictive ACL on Windows (SYSTEM + Administrators,
+inherited ACEs stripped) on the `certs` directory and file, and mode `0600` on
+non-Windows. The write is atomic (hardened temp file, then move). The persisted
+certificate is reused across restarts (stable thumbprint, so CA-pinning clients
+keep working) and regenerated only when it is missing, expired (and
+`RegenerateIfExpired` is `true`), or unreadable/corrupt. If the directory is not
+writable or the ACL cannot be applied, the gateway fails fast with a diagnostic
+naming the path rather than falling back to an in-memory certificate.
+
+**Logging.** On generate or load, the gateway logs the certificate thumbprint,
+SAN list, and `notAfter` at Information. The PFX bytes, export password, and
+private key are never logged.
+
+**Operator override.** The generated certificate is only the HTTPS *default*. To
+use a real certificate, configure one explicitly — either per endpoint via
+`Kestrel:Endpoints:<name>:Certificate` (`Path`/`Subject`/`Thumbprint`, etc., as
+in the table above) or globally via `Kestrel:Certificates:Default`. An
+explicitly-configured certificate takes precedence, and the gateway then writes
+no self-signed material.
+
+### Client side
+
+Each official client opts into TLS explicitly. For the .NET client
+(`MxGatewayClientOptions`):
+
+| Option | Effect |
+|---|---|
+| `UseTls` (default `false`) | Enables TLS. Requires an `https://` endpoint; an `https://` endpoint without `UseTls` fails validation, and vice versa. |
+| `CaCertificatePath` | Pins a custom root (self-signed / private CA) using `CustomRootTrust` chain validation instead of the OS trust store; the .NET client also enforces the certificate hostname/SAN match on this path. |
+| `RequireCertificateValidation` (default `false`) | Forces OS/system-trust verification on a TLS connection with no pinned CA. Leave `false` for the lenient default. |
+| `ServerNameOverride` | SNI / certificate host name override when the dialed host differs from the certificate CN/SAN. |
+
+To pair with the auto-generated self-signed certificate above, the clients are
+**lenient by default**: a TLS connection with no pinned CA accepts whatever
+certificate the gateway presents. Pin `CaCertificatePath` to verify, or set
+`RequireCertificateValidation` to force system-trust verification without
+pinning. The other language clients expose the equivalent options; the exact
+behavior differs per stack — Python uses trust-on-first-use and Rust is pin-only.
+See each client README for the as-built behavior.
+
+### Gateway↔worker IPC
+
+Transport security here applies only to the public gRPC channel. The
+gateway↔worker link is a per-session **named pipe**
+(`mxaccess-gateway-{gatewayPid}-{sessionId}`), not a network socket. It is not
+TLS-encrypted and does not need to be: it never leaves the local Windows host and
+is secured by the OS pipe ACL. See [Worker Frame Protocol](./WorkerFrameProtocol.md).
+
 ## Related Documentation

 - [Gateway Process Detailed Design](./GatewayProcessDesign.md)
@@ -243,9 +243,27 @@ services.AddGrpc(options => options.Interceptors.Add<GatewayGrpcAuthorizationInt

 Because the interceptor runs before any handler, `MxAccessGatewayService` can safely assume the call has been authorized and that `IGatewayRequestIdentityAccessor.Current` is populated. The handler's only responsibility is to read the identity for `OpenSession` so the session is owned by the authenticated principal; it does not perform any authorization checks of its own. See [Authorization](./Authorization.md) for the policy and identity model.

+## Transport Security
+
+The gRPC endpoint runs over HTTP/2, in cleartext (`h2c`) or TLS depending on the
+Kestrel endpoint configuration. The current deployments serve it in cleartext, so
+the API key and request payloads cross the network unencrypted. The endpoint,
+protocol pinning, and TLS certificate configuration — plus the corresponding
+client `UseTls` / `CaCertificatePath` options — are documented in
+[Host Endpoints and Transport Security](./GatewayConfiguration.md#host-endpoints-and-transport-security-kestrel).
+
+To make TLS usable without PKI, the gateway can auto-generate and persist a
+self-signed certificate when an HTTPS endpoint is configured without one, and the
+language clients are lenient by default — a TLS connection with no pinned CA
+accepts the presented certificate (with per-stack nuances: Python is
+trust-on-first-use, Rust is pin-only). See
+[Automatic self-signed certificate](./GatewayConfiguration.md#automatic-self-signed-certificate)
+and each client README for the as-built behavior.
+
 ## Related Documentation

 - [Contracts](./Contracts.md)
 - [Sessions](./Sessions.md)
 - [Authorization](./Authorization.md)
+- [Gateway Configuration](./GatewayConfiguration.md)
 - [Gateway Process Design](./GatewayProcessDesign.md)
@@ -0,0 +1,156 @@
+# Gateway TLS Auto-Certificate and Lenient Client Trust — Design
+
+Date: 2026-06-01
+Status: Approved (brainstorming), pending implementation plan
+
+## Problem
+
+The gateway can serve gRPC and the dashboard over TLS, but only if an operator
+supplies a certificate via the Kestrel `https://` endpoint config. With no cert,
+an `https` endpoint fails at startup with Kestrel's opaque "No server certificate
+was specified" error. Both current deployments therefore run plaintext (`h2c`),
+exposing the API key and request payloads on the wire.
+
+`mxaccessgw` is an internal tool. The goal is for TLS to "just work" with zero PKI
+management: the gateway fabricates its own long-lived certificate when an HTTPS
+endpoint is configured without one, and clients accept whatever certificate is
+presented unless an operator explicitly opts into pinning.
+
+## Decisions
+
+1. **Gateway = fill-missing-cert-only.** No new "enable TLS" switch. TLS is still
+   driven by configuring a Kestrel `https://` endpoint. New behavior: when an
+   HTTPS endpoint has no `Certificate` section, the gateway generates/loads a
+   persisted self-signed cert instead of failing. Plaintext-only hosts are
+   untouched — no certificate or key material is ever written for them.
+2. **Persist & reuse.** The self-signed cert is saved as a PFX under
+   `C:\ProgramData\MxGateway\certs`, reused across restarts, regenerated only if
+   missing, expired, or unreadable. Stable thumbprint; survives restarts; any
+   CA-pinning client keeps working.
+3. **Clients = lenient TLS, plaintext default.** When a client connects over TLS
+   without a pinned CA, it skips verification (accepts any cert). Pinning a CA file
+   restores full verification. The per-client connection default (mostly
+   plaintext/`http`) does not change — TLS is still opt-in via the endpoint scheme.
+
+**Scope boundary:** the gateway↔worker named-pipe IPC is unchanged (local,
+OS-secured by the pipe ACL). This work touches only the public gRPC/dashboard
+transport and the five language clients.
+
+## Gateway component
+
+New type `SelfSignedCertificateProvider` in
+`src/ZB.MOM.WW.MxGateway.Server/Security/Tls/`.
+
+1. **Detect need.** Inspect `Kestrel:Endpoints:*` configuration at startup. If any
+   endpoint has an `https://` URL and no `Certificate` subsection, a default cert
+   is needed. If none do, the provider is a no-op (no file written).
+2. **Load-or-create.** Look for the persisted PFX. If present, valid, and
+   unexpired, load it. Otherwise generate and persist.
+3. **Generate.** `CertificateRequest` with **ECDSA P-256**, `notBefore = now - 1
+   day` (clock-skew slack), `notAfter = now + ValidityYears`. SANs: `DNS=localhost`,
+   `DNS=<MachineName>`, `DNS=<MachineName.FQDN>` when resolvable, plus
+   `IP=127.0.0.1` and `IP=::1`. Server-auth EKU.
+4. **Persist securely.** Write the PFX with an **empty** export password (a random
+   in-memory password cannot be reused across restarts, which the persist-and-reuse
+   decision requires); protect the private key with a restrictive ACL (SYSTEM +
+   Administrators + service account) on the `certs` directory and file on Windows,
+   and `0600` on non-Windows; atomic write (temp + rename). After generating, the
+   cert is reloaded from the persisted PFX so Kestrel always serves the on-disk key.
+5. **Wire into Kestrel.** In `GatewayApplication.CreateBuilder`, add
+   `builder.WebHost.ConfigureKestrel(o => o.ConfigureHttpsDefaults(h =>
+   h.ServerCertificate = cert))`. `ConfigureHttpsDefaults` supplies the cert only
+   for HTTPS endpoints that did not specify their own, so an operator-configured
+   `Kestrel:Endpoints:*:Certificate` transparently overrides it. One hook covers
+   both the gRPC and dashboard ports.
+
+### New config block `MxGateway:Tls`
+
+All optional; the zero-config path needs none of them.
+
+| Option | Default | Purpose |
+|---|---|---|
+| `Tls:SelfSignedCertPath` | `C:\ProgramData\MxGateway\certs\gateway-selfsigned.pfx` | Where the generated cert lives |
+| `Tls:ValidityYears` | `10` | Lifetime of the generated cert |
+| `Tls:AdditionalDnsNames` | `[]` | Extra SANs (e.g. a load-balancer name) |
+| `Tls:RegenerateIfExpired` | `true` | Auto-replace an expired persisted cert |
+
+Validated by `GatewayOptionsValidator`: `ValidityYears` in 1–100,
+`SelfSignedCertPath` is a valid path shape when non-blank, and
+`AdditionalDnsNames` entries are non-blank. (The "https endpoint exists but cert
+path is blank" fail-fast lives in the bootstrap/provider, not the validator,
+because the validator only sees the `MxGateway` section, not `Kestrel:Endpoints`.)
+
+**Logging:** on generate/load, log thumbprint + SAN list + `notAfter` at
+Information. Never log the PFX password or private key.
+
+## Client lenient-TLS behavior
+
+Uniform rule: **TLS on + no CA pinned ⇒ skip verification; CA pinned ⇒ full
+verification.** No transport default changes. Each client also exposes an explicit
+switch to force-disable leniency (strict-without-pinning) for the future.
+
+| Client | Mechanism | Effort |
+|---|---|---|
+| .NET | In `CreateHttpHandler`, when `UseTls` and `CaCertificatePath` empty, set `SslOptions.RemoteCertificateValidationCallback = (_,_,_,_) => true`. CA path keeps existing custom-root validation. | trivial |
+| Go | In `buildCredentials`, when TLS and no `CACertFile`/`TLSConfig`, use `tls.Config{InsecureSkipVerify: true, ServerName: override}`. | trivial |
+| Java | grpc-netty-shaded 1.76.0 ships `InsecureTrustManagerFactory`. When TLS and no CA, build `GrpcSslContexts.forClient().trustManager(InsecureTrustManagerFactory.INSTANCE)`. | easy |
+| Python | grpc-python has no per-channel skip-verify. Fetch the server leaf cert at connect via `ssl.get_server_certificate((host, port))`, pass it as `root_certificates` to `ssl_channel_credentials`, plus `grpc.ssl_target_name_override`. Effectively trusts what is presented (TOFU). | moderate, special-cased |
+| Rust | tonic 0.13.1 + rustls (`tls-ring`). Implement a custom `rustls::client::danger::ServerCertVerifier` that accepts everything, build a `rustls::ClientConfig` via `.dangerous().with_custom_certificate_verifier(...)`, feed it to the channel. May require a custom hyper-rustls connector if `ClientTlsConfig` will not take a raw rustls config. **Needs an API spike.** | highest |
+
+### Honesty caveats
+
+- **Python** is not literally "ignore the cert"; it pins whatever the server
+  presents on first contact via a separate unverified TLS probe. For a self-signed
+  internal cert this is the intended outcome. Documented as a difference.
+- **Rust** leniency depends on the tonic 0.13 TLS surface. If a custom verifier is
+  disproportionately invasive, the fallback is to require a CA file for Rust TLS
+  (pin-only) and document Rust as the exception.
+
+## Error handling
+
+Gateway:
+- Cert dir not writable / ACL fails ⇒ fail fast at startup with a diagnostic naming
+  the path and required permission. No silent in-memory fallback.
+- Persisted PFX corrupt/unreadable ⇒ warn, regenerate, overwrite.
+- Persisted cert expired ⇒ regenerate if `RegenerateIfExpired` (default), else fail
+  fast instructing the operator to delete it or enable regeneration.
+- HTTPS endpoint configured but generation disabled / path empty ⇒ validator
+  rejects at startup rather than letting Kestrel throw its opaque error.
+
+Clients: surface unchanged. Skip-verify cannot itself raise. Python's pre-fetch
+wraps connect failure into the existing connect-error type with the endpoint in the
+message. Rust pin-only fallback surfaces the existing CA-file error.
+
+## Documentation (same commit as source, per CLAUDE.md)
+
+- `docs/GatewayConfiguration.md` — extend the TLS section: auto-generation, the
+  `MxGateway:Tls:*` block, persistence location/ACL, thumbprint logging, operator
+  override via `Kestrel:Endpoints:*:Certificate`.
+- Each client README + `*ClientDesign.md` — "TLS is lenient by default; pin a CA to
+  verify," with Python TOFU and any Rust caveat noted.
+- `docs/DesignDecisions.md` — record both posture choices and the why (internal
+  tool, no PKI) so they are not mistaken for an oversight.
+
+## Testing
+
+Gateway (`MxGateway.Tests`, no MXAccess):
+- `SelfSignedCertificateProvider`: SANs, server-auth EKU, `notAfter ≈ now +
+  ValidityYears`, ECDSA P-256.
+- Load-or-create: valid persisted PFX reused (same thumbprint); expired regenerates
+  when enabled; corrupt regenerates with a warning.
+- Detection: HTTPS-without-cert engages; all-plaintext no-ops and writes no file;
+  endpoint with its own cert is not overridden.
+- `GatewayOptionsValidator`: new `Tls:*` rules.
+- Host integration: `Kestrel:Endpoints:Http:Url=https://127.0.0.1:0` builds and
+  binds (today it throws "no certificate specified").
+
+Clients: each test project gets a lenient-TLS test against a throwaway self-signed
+cert — connect with no CA succeeds; pinning a wrong CA fails (proves pinning still
+verifies). Python exercises the pre-fetch path; mark opt-in if loopback timing is
+flaky. Standard (non-live) tests; no MXAccess or external services.
+
+Cross-language: add a TLS variant note to `docs/CrossLanguageSmokeMatrix.md`;
+running the matrix over TLS stays manual/opt-in, consistent with the existing gate.
+
+Per-component verification follows CLAUDE.md's source-update table (build + test
+each touched component independently).
@@ -0,0 +1,18 @@
+{
+  "planPath": "docs/plans/2026-06-01-gateway-cert-autogen-implementation.md",
+  "tasks": [
+    {"id": 1, "subject": "Task 1: Add TlsOptions config + bind into GatewayOptions", "status": "pending"},
+    {"id": 2, "subject": "Task 2: Validate MxGateway:Tls in GatewayOptionsValidator", "status": "pending", "blockedBy": [1]},
+    {"id": 3, "subject": "Task 3: SelfSignedCertificateProvider.GenerateCertificate", "status": "pending", "blockedBy": [1]},
+    {"id": 4, "subject": "Task 4: SelfSignedCertificateProvider.LoadOrCreate (persist/reuse/regenerate/ACL)", "status": "pending", "blockedBy": [3]},
+    {"id": 5, "subject": "Task 5: KestrelTlsInspector (detect HTTPS-without-cert)", "status": "pending"},
+    {"id": 6, "subject": "Task 6: Wire auto-cert into GatewayApplication.CreateBuilder", "status": "pending", "blockedBy": [1, 4, 5]},
+    {"id": 7, "subject": "Task 7: .NET client lenient TLS by default", "status": "pending"},
+    {"id": 8, "subject": "Task 8: Go client lenient TLS by default", "status": "pending"},
+    {"id": 9, "subject": "Task 9: Java client lenient TLS by default", "status": "pending"},
+    {"id": 10, "subject": "Task 10: Python client lenient TLS via TOFU pre-fetch", "status": "pending"},
+    {"id": 11, "subject": "Task 11: Rust client lenient TLS via rustls verifier (spike + fallback)", "status": "pending"},
+    {"id": 12, "subject": "Task 12: Documentation", "status": "pending", "blockedBy": [6, 7, 8, 9, 10, 11]}
+  ],
+  "lastUpdated": "2026-06-01"
+}
@@ -43,4 +43,7 @@ public sealed class GatewayOptions
    /// behaviour (alarms disabled).
    /// </summary>
    public AlarmsOptions Alarms { get; init; } = new();
+
+    /// <summary>Gets self-signed TLS certificate auto-generation options.</summary>
+    public TlsOptions Tls { get; init; } = new();
 }
@@ -26,6 +26,7 @@ public sealed class GatewayOptionsValidator : IValidateOptions<GatewayOptions>
        ValidateDashboard(options.Dashboard, failures);
        ValidateProtocol(options.Protocol, failures);
        ValidateAlarms(options.Alarms, failures);
+        ValidateTls(options.Tls, failures);

        return failures.Count == 0
            ? ValidateOptionsResult.Success
@@ -262,6 +263,36 @@ public sealed class GatewayOptionsValidator : IValidateOptions<GatewayOptions>
        }
    }

+    private const int MinimumCertValidityYears = 1;
+    private const int MaximumCertValidityYears = 100;
+
+    private static void ValidateTls(TlsOptions options, List<string> failures)
+    {
+        if (options.ValidityYears is < MinimumCertValidityYears or > MaximumCertValidityYears)
+        {
+            failures.Add(
+                $"MxGateway:Tls:ValidityYears must be between {MinimumCertValidityYears} and {MaximumCertValidityYears}.");
+        }
+
+        // The default is non-blank, so this only catches an explicitly-blanked path.
+        AddIfBlank(
+            options.SelfSignedCertPath,
+            "MxGateway:Tls:SelfSignedCertPath must not be blank.",
+            failures);
+        AddIfInvalidPath(
+            options.SelfSignedCertPath,
+            "MxGateway:Tls:SelfSignedCertPath must be a valid filesystem path.",
+            failures);
+
+        foreach (string dns in options.AdditionalDnsNames)
+        {
+            if (string.IsNullOrWhiteSpace(dns))
+            {
+                failures.Add("MxGateway:Tls:AdditionalDnsNames entries must be non-blank.");
+            }
+        }
+    }
+
    private static void ValidateProtocol(ProtocolOptions options, List<string> failures)
    {
        if (options.WorkerProtocolVersion != GatewayContractInfo.WorkerProtocolVersion)
@@ -0,0 +1,22 @@
+namespace ZB.MOM.WW.MxGateway.Server.Configuration;
+
+/// <summary>
+/// Options controlling the gateway's self-signed certificate auto-generation.
+/// Only consulted when a Kestrel HTTPS endpoint is configured without its own
+/// certificate; plaintext deployments never trigger generation.
+/// </summary>
+public sealed class TlsOptions
+{
+    /// <summary>Path to the persisted self-signed PFX. Reused across restarts.</summary>
+    public string SelfSignedCertPath { get; init; } =
+        @"C:\ProgramData\MxGateway\certs\gateway-selfsigned.pfx";
+
+    /// <summary>Lifetime in years of a freshly generated certificate.</summary>
+    public int ValidityYears { get; init; } = 10;
+
+    /// <summary>Extra DNS SANs to embed (e.g. a load-balancer name).</summary>
+    public IReadOnlyList<string> AdditionalDnsNames { get; init; } = [];
+
+    /// <summary>Regenerate the persisted certificate when it has expired.</summary>
+    public bool RegenerateIfExpired { get; init; } = true;
+}
@@ -1,4 +1,7 @@
+using System.Security.Cryptography.X509Certificates;
 using Microsoft.AspNetCore.Hosting.StaticWebAssets;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Configuration;
 using ZB.MOM.WW.MxGateway.Contracts;
 using ZB.MOM.WW.MxGateway.Server.Alarms;
 using ZB.MOM.WW.MxGateway.Server.Configuration;
@@ -55,6 +58,8 @@ public static class GatewayApplication
        });
        StaticWebAssetsLoader.UseStaticWebAssets(builder.Environment, builder.Configuration);

+        ConfigureSelfSignedTls(builder);
+
        builder.Services.AddGatewayConfiguration();
        builder.Services.AddSqliteAuthStore();
        builder.Services.AddGatewayGrpcAuthorization();
@@ -72,6 +77,34 @@ public static class GatewayApplication
        return builder;
    }

+    private static void ConfigureSelfSignedTls(WebApplicationBuilder builder)
+    {
+        if (!Security.Tls.KestrelTlsInspector.RequiresGeneratedCertificate(builder.Configuration))
+        {
+            return;
+        }
+
+        Configuration.TlsOptions tlsOptions =
+            builder.Configuration.GetSection("MxGateway:Tls").Get<Configuration.TlsOptions>()
+            ?? new Configuration.TlsOptions();
+
+        using ILoggerFactory loggerFactory = LoggerFactory.Create(logging =>
+        {
+            logging.AddConfiguration(builder.Configuration.GetSection("Logging"));
+            logging.AddConsole();
+        });
+        Security.Tls.SelfSignedCertificateProvider provider = new(
+            tlsOptions,
+            loggerFactory.CreateLogger<Security.Tls.SelfSignedCertificateProvider>(),
+            TimeProvider.System);
+
+        X509Certificate2 certificate = provider.LoadOrCreate();
+        builder.WebHost.ConfigureKestrel(options =>
+            // The certificate is intentionally owned by Kestrel for the application
+            // lifetime; it is not disposed here.
+            options.ConfigureHttpsDefaults(https => https.ServerCertificate = certificate));
+    }
+
    private static string ResolveContentRootPath()
    {
        string? configuredContentRootPath = Environment.GetEnvironmentVariable("ASPNETCORE_CONTENTROOT");
@@ -0,0 +1,51 @@
+using Microsoft.Extensions.Configuration;
+
+namespace ZB.MOM.WW.MxGateway.Server.Security.Tls;
+
+/// <summary>
+/// Inspects the Kestrel configuration to decide whether the gateway must supply
+/// a generated default certificate (an HTTPS endpoint exists with no certificate
+/// of its own).
+/// </summary>
+public static class KestrelTlsInspector
+{
+    /// <summary>
+    /// Returns <see langword="true"/> when at least one HTTPS endpoint in
+    /// <c>Kestrel:Endpoints</c> has no certificate of its own (no
+    /// <c>Certificate:Path</c>, <c>Certificate:Subject</c>, or
+    /// <c>Certificate:Thumbprint</c>), meaning the gateway must supply a
+    /// generated fallback certificate.
+    /// </summary>
+    public static bool RequiresGeneratedCertificate(IConfiguration configuration)
+    {
+        // A Kestrel default certificate applies to every endpoint that lacks its own.
+        // If the operator configured one, the gateway must not override it.
+        if (HasConfiguredCertificate(configuration.GetSection("Kestrel:Certificates:Default")))
+        {
+            return false;
+        }
+
+        IConfigurationSection endpoints = configuration.GetSection("Kestrel:Endpoints");
+        foreach (IConfigurationSection endpoint in endpoints.GetChildren())
+        {
+            string? url = endpoint["Url"];
+            if (string.IsNullOrWhiteSpace(url) ||
+                !url.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
+            {
+                continue;
+            }
+
+            if (!HasConfiguredCertificate(endpoint.GetSection("Certificate")))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private static bool HasConfiguredCertificate(IConfigurationSection certificate)
+        => !string.IsNullOrWhiteSpace(certificate["Path"]) ||
+           !string.IsNullOrWhiteSpace(certificate["Subject"]) ||
+           !string.IsNullOrWhiteSpace(certificate["Thumbprint"]);
+}
@@ -0,0 +1,239 @@
+using System.Net;
+using System.Net.Sockets;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.MxGateway.Server.Configuration;
+
+namespace ZB.MOM.WW.MxGateway.Server.Security.Tls;
+
+/// <summary>
+/// Generates and persists a long-lived self-signed certificate used as the
+/// Kestrel HTTPS default when no operator certificate is configured.
+/// </summary>
+public sealed class SelfSignedCertificateProvider
+{
+    private const string ServerAuthOid = "1.3.6.1.5.5.7.3.1";
+    private const string SubjectAltNameOid = "2.5.29.17";
+
+    private readonly TlsOptions _options;
+    private readonly ILogger<SelfSignedCertificateProvider> _logger;
+    private readonly TimeProvider _timeProvider;
+
+    public SelfSignedCertificateProvider(
+        TlsOptions options,
+        ILogger<SelfSignedCertificateProvider> logger,
+        TimeProvider timeProvider)
+    {
+        _options = options;
+        _logger = logger;
+        _timeProvider = timeProvider;
+    }
+
+    /// <summary>Creates a fresh in-memory ECDSA P-256 self-signed certificate.</summary>
+    public X509Certificate2 GenerateCertificate()
+    {
+        using ECDsa key = ECDsa.Create(ECCurve.NamedCurves.nistP256);
+        CertificateRequest request = new(
+            new X500DistinguishedName("CN=MxAccessGateway Self-Signed"),
+            key,
+            HashAlgorithmName.SHA256);
+
+        // End-entity (non-CA) certificate: RFC 5280 marks BasicConstraints critical only for CA certs.
+        request.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, false));
+        request.CertificateExtensions.Add(new X509KeyUsageExtension(
+            X509KeyUsageFlags.DigitalSignature,
+            critical: true));
+        request.CertificateExtensions.Add(new X509EnhancedKeyUsageExtension(
+            [new Oid(ServerAuthOid, "Server Authentication")],
+            critical: false));
+
+        SubjectAlternativeNameBuilder san = new();
+        san.AddDnsName("localhost");
+        string machine = Environment.MachineName;
+        if (!string.IsNullOrWhiteSpace(machine))
+        {
+            san.AddDnsName(machine);
+        }
+
+        // Best-effort: add the machine FQDN when it differs from the short name and "localhost".
+        // GetHostEntry may fail if DNS is unavailable; skip silently in that case.
+        try
+        {
+            string fqdn = Dns.GetHostEntry(machine).HostName;
+            if (!string.IsNullOrWhiteSpace(fqdn)
+                && !fqdn.Equals("localhost", StringComparison.OrdinalIgnoreCase)
+                && !fqdn.Equals(machine, StringComparison.OrdinalIgnoreCase))
+            {
+                san.AddDnsName(fqdn);
+            }
+        }
+        catch (SocketException) { /* DNS not resolvable — FQDN SAN is optional */ }
+        catch (ArgumentException) { /* invalid host name — skip */ }
+
+        foreach (string extra in _options.AdditionalDnsNames)
+        {
+            if (!string.IsNullOrWhiteSpace(extra))
+            {
+                san.AddDnsName(extra);
+            }
+        }
+
+        san.AddIpAddress(IPAddress.Loopback);
+        san.AddIpAddress(IPAddress.IPv6Loopback);
+        request.CertificateExtensions.Add(san.Build());
+
+        DateTimeOffset now = _timeProvider.GetUtcNow();
+        return request.CreateSelfSigned(now.AddDays(-1), now.AddYears(_options.ValidityYears));
+    }
+
+    /// <summary>Loads the persisted certificate, regenerating when missing,
+    /// expired (and allowed), or unreadable.</summary>
+    public X509Certificate2 LoadOrCreate()
+    {
+        string path = _options.SelfSignedCertPath;
+        if (string.IsNullOrWhiteSpace(path))
+        {
+            throw new InvalidOperationException(
+                "MxGateway:Tls:SelfSignedCertPath must be set when an HTTPS endpoint has no certificate.");
+        }
+
+        if (File.Exists(path))
+        {
+            try
+            {
+                X509Certificate2 existing = X509CertificateLoader.LoadPkcs12FromFile(path, password: null, KeyStorageFlags());
+                if (existing.NotAfter.ToUniversalTime() > _timeProvider.GetUtcNow().UtcDateTime)
+                {
+                    Log("Loaded", existing);
+                    return existing;
+                }
+
+                if (!_options.RegenerateIfExpired)
+                {
+                    string notAfter = existing.NotAfter.ToUniversalTime().ToString("u");
+                    existing.Dispose();
+                    throw new InvalidOperationException(
+                        $"Persisted gateway certificate at '{path}' expired on {notAfter} " +
+                        "and MxGateway:Tls:RegenerateIfExpired is false.");
+                }
+
+                _logger.LogWarning(
+                    "Persisted gateway certificate at {Path} expired on {NotAfter:u}; regenerating.",
+                    path, existing.NotAfter.ToUniversalTime());
+                existing.Dispose();
+            }
+            catch (CryptographicException ex)
+            {
+                _logger.LogWarning(ex,
+                    "Persisted gateway certificate at {Path} is unreadable; regenerating.", path);
+            }
+            catch (Exception ex) when (ex is UnauthorizedAccessException or IOException)
+            {
+                // A permission/IO error must fail fast: do not regenerate (which would
+                // overwrite the operator's file), surface the cause instead.
+                throw new InvalidOperationException(
+                    $"Persisted gateway certificate at '{path}' could not be read; the gateway " +
+                    "process lacks read access or the file is otherwise inaccessible.", ex);
+            }
+        }
+
+        return GenerateAndPersist(path);
+    }
+
+    private X509Certificate2 GenerateAndPersist(string path)
+    {
+        using X509Certificate2 generated = GenerateCertificate();
+        byte[] pfx = generated.Export(X509ContentType.Pkcs12);
+        try
+        {
+            string? directory = Path.GetDirectoryName(path);
+            if (!string.IsNullOrEmpty(directory))
+            {
+                Directory.CreateDirectory(directory);
+            }
+
+            // The private-key bytes must never touch a world-readable file. Create the
+            // temp file empty, harden its permissions, and only then write the PFX into
+            // the already-protected file. The temp path is in the same directory as the
+            // target so the Move is atomic and preserves the hardened DACL/mode.
+            string temp = path + ".tmp";
+            using (File.Create(temp)) { }
+            HardenPermissions(temp);
+
+            // Writing into an existing file truncates content but preserves its ACL/mode.
+            // If the write or move fails the hardened temp file (which may contain private-key
+            // material) must not be left on disk; delete it best-effort before rethrowing.
+            try
+            {
+                File.WriteAllBytes(temp, pfx);
+                File.Move(temp, path, overwrite: true);
+                HardenPermissions(path);
+            }
+            catch (Exception)
+            {
+                try { File.Delete(temp); } catch { /* best effort */ }
+                throw;
+            }
+
+            X509Certificate2 loaded = X509CertificateLoader.LoadPkcs12FromFile(path, password: null, KeyStorageFlags());
+            Log("Generated", loaded);
+            return loaded;
+        }
+        finally
+        {
+            // pfx holds raw private-key material; clear it as soon as it is no longer needed.
+            Array.Clear(pfx, 0, pfx.Length);
+        }
+    }
+
+    private static X509KeyStorageFlags KeyStorageFlags()
+        => OperatingSystem.IsWindows()
+            ? X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable
+            : X509KeyStorageFlags.Exportable;
+
+    private static void HardenPermissions(string path)
+    {
+        if (OperatingSystem.IsWindows())
+        {
+            HardenWindowsAcl(path);
+        }
+        else
+        {
+            File.SetUnixFileMode(path, UnixFileMode.UserRead | UnixFileMode.UserWrite);
+        }
+    }
+
+    [System.Runtime.Versioning.SupportedOSPlatform("windows")]
+    private static void HardenWindowsAcl(string path)
+    {
+        FileInfo file = new(path);
+        System.Security.AccessControl.FileSecurity security = new();
+        security.SetAccessRuleProtection(isProtected: true, preserveInheritance: false);
+
+        foreach (System.Security.Principal.WellKnownSidType sid in new[]
+        {
+            System.Security.Principal.WellKnownSidType.LocalSystemSid,
+            System.Security.Principal.WellKnownSidType.BuiltinAdministratorsSid,
+        })
+        {
+            System.Security.Principal.SecurityIdentifier identifier = new(sid, null);
+            security.AddAccessRule(new System.Security.AccessControl.FileSystemAccessRule(
+                identifier,
+                System.Security.AccessControl.FileSystemRights.FullControl,
+                System.Security.AccessControl.AccessControlType.Allow));
+        }
+
+        file.SetAccessControl(security);
+    }
+
+    private void Log(string action, X509Certificate2 cert)
+    {
+        string sans = cert.Extensions
+            .FirstOrDefault(e => e.Oid?.Value == SubjectAltNameOid)?
+            .Format(false) ?? "(none)";
+        _logger.LogInformation(
+            "{Action} gateway self-signed certificate: thumbprint={Thumbprint}, notAfter={NotAfter:u}, sans={Sans}",
+            action, cert.Thumbprint, cert.NotAfter.ToUniversalTime(), sans);
+    }
+}
@@ -0,0 +1,68 @@
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.MxGateway.Server.Configuration;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Configuration;
+
+public sealed class GatewayOptionsValidatorTests
+{
+    // Constructs the minimal valid GatewayOptions by relying on each sub-option's
+    // design-default values; those defaults are validated separately in GatewayOptionsTests.
+    private static GatewayOptions ValidOptions() => new();
+
+    private static GatewayOptions CloneWithTls(GatewayOptions source, TlsOptions tls)
+        => new()
+        {
+            Authentication = source.Authentication,
+            Ldap = source.Ldap,
+            Worker = source.Worker,
+            Sessions = source.Sessions,
+            Events = source.Events,
+            Dashboard = source.Dashboard,
+            Protocol = source.Protocol,
+            Alarms = source.Alarms,
+            Tls = tls,
+        };
+
+    [Fact]
+    public void Validate_Succeeds_WithDefaultTlsOptions()
+    {
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, ValidOptions());
+        Assert.True(result.Succeeded);
+    }
+
+    [Fact]
+    public void Validate_Fails_WhenTlsValidityYearsOutOfRange()
+    {
+        GatewayOptions withBadTls = CloneWithTls(ValidOptions(), new TlsOptions { ValidityYears = 0 });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, withBadTls);
+        Assert.True(result.Failed);
+        Assert.Contains(result.Failures!, f => f.Contains("MxGateway:Tls:ValidityYears"));
+    }
+
+    [Fact]
+    public void Validate_Fails_WhenTlsValidityYearsTooLarge()
+    {
+        GatewayOptions withBadTls = CloneWithTls(ValidOptions(), new TlsOptions { ValidityYears = 101 });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, withBadTls);
+        Assert.True(result.Failed);
+        Assert.Contains(result.Failures!, f => f.Contains("MxGateway:Tls:ValidityYears"));
+    }
+
+    [Fact]
+    public void Validate_Fails_WhenAdditionalDnsNameBlank()
+    {
+        GatewayOptions options = CloneWithTls(ValidOptions(), new TlsOptions { AdditionalDnsNames = [" "] });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Failed);
+        Assert.Contains(result.Failures!, f => f.Contains("MxGateway:Tls:AdditionalDnsNames"));
+    }
+
+    [Fact]
+    public void Validate_Fails_WhenSelfSignedCertPathBlank()
+    {
+        GatewayOptions options = CloneWithTls(ValidOptions(), new TlsOptions { SelfSignedCertPath = "  " });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Failed);
+        Assert.Contains(result.Failures!, f => f.Contains("MxGateway:Tls:SelfSignedCertPath must not be blank."));
+    }
+}
@@ -0,0 +1,39 @@
+using Microsoft.Extensions.Configuration;
+using ZB.MOM.WW.MxGateway.Server.Configuration;
+using Xunit;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Configuration;
+
+public sealed class TlsOptionsBindingTests
+{
+    [Fact]
+    public void Defaults_AreApplied_WhenSectionAbsent()
+    {
+        TlsOptions options = new();
+        Assert.Equal(10, options.ValidityYears);
+        Assert.True(options.RegenerateIfExpired);
+        Assert.Empty(options.AdditionalDnsNames);
+        Assert.False(string.IsNullOrWhiteSpace(options.SelfSignedCertPath));
+    }
+
+    [Fact]
+    public void Binds_FromMxGatewayTlsSection()
+    {
+        IConfiguration config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["MxGateway:Tls:ValidityYears"] = "5",
+                ["MxGateway:Tls:SelfSignedCertPath"] = @"C:\tmp\gw.pfx",
+                ["MxGateway:Tls:RegenerateIfExpired"] = "false",
+                ["MxGateway:Tls:AdditionalDnsNames:0"] = "gw.internal",
+            })
+            .Build();
+
+        GatewayOptions options = config.GetSection(GatewayOptions.SectionName).Get<GatewayOptions>()!;
+
+        Assert.Equal(5, options.Tls.ValidityYears);
+        Assert.Equal(@"C:\tmp\gw.pfx", options.Tls.SelfSignedCertPath);
+        Assert.False(options.Tls.RegenerateIfExpired);
+        Assert.Equal("gw.internal", Assert.Single(options.Tls.AdditionalDnsNames));
+    }
+}
@@ -0,0 +1,72 @@
+using System.Net.Security;
+using System.Net.Sockets;
+using System.Security.Cryptography.X509Certificates;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.Extensions.DependencyInjection;
+using ZB.MOM.WW.MxGateway.Server;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Gateway;
+
+public sealed class GatewayTlsBootstrapTests
+{
+    /// <summary>
+    /// Verifies that when a Kestrel HTTPS endpoint is configured without its own certificate,
+    /// the gateway supplies the generated self-signed certificate as the Kestrel HTTPS default.
+    /// The host must start and bind, and the certificate served on the TLS handshake must be the
+    /// gateway's generated cert (subject <c>CN=MxAccessGateway Self-Signed</c>) — not an ambient
+    /// ASP.NET Core development certificate. On a host with no dev cert installed, starting a
+    /// cert-less HTTPS endpoint throws "No server certificate was specified"; on a host that has a
+    /// trusted dev cert, Kestrel would otherwise serve that dev cert (<c>CN=localhost</c>), so the
+    /// subject assertion is what makes this test fail without the wiring on either kind of host.
+    /// </summary>
+    [Fact]
+    public async Task Host_ServesGeneratedCertificate_WhenHttpsEndpointHasNoCertificate()
+    {
+        string certDir = Directory.CreateTempSubdirectory().FullName;
+        try
+        {
+            Environment.SetEnvironmentVariable("Kestrel__Endpoints__Test__Url", "https://127.0.0.1:0");
+            Environment.SetEnvironmentVariable(
+                "MxGateway__Tls__SelfSignedCertPath", Path.Combine(certDir, "gw.pfx"));
+
+            WebApplication app = GatewayApplication.Build([]);
+            await app.StartAsync();
+            try
+            {
+                string servedSubject = await ReadServedCertificateSubjectAsync(app);
+                Assert.Contains("MxAccessGateway Self-Signed", servedSubject, StringComparison.Ordinal);
+            }
+            finally
+            {
+                await app.StopAsync();
+                await app.DisposeAsync();
+            }
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable("Kestrel__Endpoints__Test__Url", null);
+            Environment.SetEnvironmentVariable("MxGateway__Tls__SelfSignedCertPath", null);
+            Directory.Delete(certDir, recursive: true);
+        }
+    }
+
+    private static async Task<string> ReadServedCertificateSubjectAsync(WebApplication app)
+    {
+        IServerAddressesFeature addresses =
+            app.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()
+            ?? throw new InvalidOperationException("Server addresses feature was not available.");
+        Uri endpoint = new(addresses.Addresses.First());
+
+        using TcpClient client = new();
+        await client.ConnectAsync(endpoint.Host, endpoint.Port);
+        using SslStream ssl = new(
+            client.GetStream(),
+            leaveInnerStreamOpen: false,
+            userCertificateValidationCallback: (_, _, _, _) => true);
+        await ssl.AuthenticateAsClientAsync("127.0.0.1");
+
+        return ssl.RemoteCertificate?.Subject ?? "(none)";
+    }
+}
@@ -0,0 +1,68 @@
+using Microsoft.Extensions.Configuration;
+using ZB.MOM.WW.MxGateway.Server.Security.Tls;
+using Xunit;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Security.Tls;
+
+public sealed class KestrelTlsInspectorTests
+{
+    private static IConfiguration Config(params (string Key, string Value)[] entries)
+        => new ConfigurationBuilder()
+            .AddInMemoryCollection(entries.ToDictionary(e => e.Key, e => (string?)e.Value))
+            .Build();
+
+    [Fact]
+    public void RequiresGeneratedCertificate_True_WhenHttpsEndpointHasNoCertificate()
+        => Assert.True(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(("Kestrel:Endpoints:Http:Url", "https://0.0.0.0:5120"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_False_WhenAllEndpointsPlaintext()
+        => Assert.False(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(("Kestrel:Endpoints:Http:Url", "http://0.0.0.0:5120"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_False_WhenHttpsEndpointHasOwnCertificate()
+        => Assert.False(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(
+                ("Kestrel:Endpoints:Http:Url", "https://0.0.0.0:5120"),
+                ("Kestrel:Endpoints:Http:Certificate:Path", @"C:\certs\real.pfx"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_False_WhenNoEndpointsConfigured()
+        => Assert.False(KestrelTlsInspector.RequiresGeneratedCertificate(Config()));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_False_WhenHttpsEndpointHasThumbprintOnly()
+        => Assert.False(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(
+                ("Kestrel:Endpoints:Https:Url", "https://0.0.0.0:5120"),
+                ("Kestrel:Endpoints:Https:Certificate:Thumbprint", "AABBCCDDEEFF00112233445566778899AABBCCDD"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_False_WhenHttpsEndpointHasSubjectOnly()
+        => Assert.False(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(
+                ("Kestrel:Endpoints:Https:Url", "https://0.0.0.0:5120"),
+                ("Kestrel:Endpoints:Https:Certificate:Subject", "CN=myserver"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_True_WhenHttpsUrlIsUppercase()
+        => Assert.True(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(("Kestrel:Endpoints:Https:Url", "HTTPS://0.0.0.0:5120"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_False_WhenKestrelDefaultCertificateConfigured()
+        => Assert.False(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(
+                ("Kestrel:Endpoints:Https:Url", "https://0.0.0.0:5120"),
+                ("Kestrel:Certificates:Default:Path", @"C:\certs\default.pfx"))));
+
+    [Fact]
+    public void RequiresGeneratedCertificate_True_WhenMixedEndpointsAndOneHttpsHasNoCert()
+        => Assert.True(KestrelTlsInspector.RequiresGeneratedCertificate(
+            Config(
+                ("Kestrel:Endpoints:Grpc:Url", "https://0.0.0.0:5120"),
+                ("Kestrel:Endpoints:Grpc:Certificate:Thumbprint", "AABBCCDDEEFF00112233445566778899AABBCCDD"),
+                ("Kestrel:Endpoints:Dashboard:Url", "https://0.0.0.0:5130"))));
+}
@@ -0,0 +1,156 @@
+using System.Security.Cryptography.X509Certificates;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Time.Testing;
+using ZB.MOM.WW.MxGateway.Server.Configuration;
+using ZB.MOM.WW.MxGateway.Server.Security.Tls;
+using Xunit;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Security.Tls;
+
+public sealed class SelfSignedCertificateProviderTests
+{
+    private static SelfSignedCertificateProvider CreateProvider(TlsOptions options, FakeTimeProvider time)
+        => new(options, NullLogger<SelfSignedCertificateProvider>.Instance, time);
+
+    [Fact]
+    public void GenerateCertificate_HasExpectedSansEkuAndValidity()
+    {
+        FakeTimeProvider time = new(new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero));
+        TlsOptions options = new() { ValidityYears = 7, AdditionalDnsNames = ["gw.internal"] };
+
+        using X509Certificate2 cert = CreateProvider(options, time).GenerateCertificate();
+
+        Assert.Equal(time.GetUtcNow().AddYears(7).UtcDateTime.Date, cert.NotAfter.ToUniversalTime().Date);
+        Assert.True(cert.NotBefore.ToUniversalTime() < time.GetUtcNow().UtcDateTime);
+        Assert.True(cert.HasPrivateKey);
+
+        string sans = ReadSubjectAltNames(cert);
+        Assert.Contains("localhost", sans);
+        Assert.Contains("gw.internal", sans);
+        Assert.Contains(Environment.MachineName, sans);
+        // Format() renders IP SANs as "IP Address:<addr>"; the IPv6 loopback may appear
+        // as "::1" or its expanded form depending on the platform crypto library.
+        Assert.Contains("127.0.0.1", sans);
+        Assert.True(sans.Contains("::1") || sans.Contains("0:0:0:0:0:0:0:1"),
+            $"Expected IPv6 loopback in SANs but got: {sans}");
+
+        X509EnhancedKeyUsageExtension eku = cert.Extensions.OfType<X509EnhancedKeyUsageExtension>().Single();
+        Assert.Contains(eku.EnhancedKeyUsages.Cast<System.Security.Cryptography.Oid>(),
+            o => o.Value == "1.3.6.1.5.5.7.3.1"); // serverAuth
+    }
+
+    [Fact]
+    public void LoadOrCreate_GeneratesPersistsAndReuses_SameThumbprint()
+    {
+        string dir = Directory.CreateTempSubdirectory().FullName;
+        try
+        {
+            string path = Path.Combine(dir, "gw.pfx");
+            FakeTimeProvider time = new(new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero));
+            TlsOptions options = new() { SelfSignedCertPath = path };
+
+            using X509Certificate2 first = CreateProvider(options, time).LoadOrCreate();
+            Assert.True(File.Exists(path));
+            using X509Certificate2 second = CreateProvider(options, time).LoadOrCreate();
+
+            Assert.Equal(first.Thumbprint, second.Thumbprint); // reused, not regenerated
+        }
+        finally { Directory.Delete(dir, recursive: true); }
+    }
+
+    [Fact]
+    public void LoadOrCreate_Regenerates_WhenPersistedCertExpired()
+    {
+        string dir = Directory.CreateTempSubdirectory().FullName;
+        try
+        {
+            string path = Path.Combine(dir, "gw.pfx");
+            FakeTimeProvider time = new(new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero));
+            TlsOptions options = new() { SelfSignedCertPath = path, ValidityYears = 1 };
+
+            using X509Certificate2 first = CreateProvider(options, time).LoadOrCreate();
+            time.Advance(TimeSpan.FromDays(800)); // past 1-year validity
+            using X509Certificate2 second = CreateProvider(options, time).LoadOrCreate();
+
+            Assert.NotEqual(first.Thumbprint, second.Thumbprint);
+        }
+        finally { Directory.Delete(dir, recursive: true); }
+    }
+
+    [Fact]
+    public void LoadOrCreate_Regenerates_WhenPersistedFileCorrupt()
+    {
+        string dir = Directory.CreateTempSubdirectory().FullName;
+        try
+        {
+            string path = Path.Combine(dir, "gw.pfx");
+            File.WriteAllText(path, "not a pfx");
+            TlsOptions options = new() { SelfSignedCertPath = path };
+            using X509Certificate2 cert = CreateProvider(options, new FakeTimeProvider()).LoadOrCreate();
+            Assert.True(cert.HasPrivateKey);
+        }
+        finally { Directory.Delete(dir, recursive: true); }
+    }
+
+    [Fact]
+    public void LoadOrCreate_Throws_WhenExpiredAndRegenerateDisabled()
+    {
+        string dir = Directory.CreateTempSubdirectory().FullName;
+        try
+        {
+            string path = Path.Combine(dir, "gw.pfx");
+            FakeTimeProvider time = new(new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero));
+            TlsOptions options = new() { SelfSignedCertPath = path, ValidityYears = 1, RegenerateIfExpired = false };
+            using (CreateProvider(options, time).LoadOrCreate()) { }
+            time.Advance(TimeSpan.FromDays(800));
+            Assert.Throws<InvalidOperationException>(() => CreateProvider(options, time).LoadOrCreate());
+        }
+        finally { Directory.Delete(dir, recursive: true); }
+    }
+
+    [Fact]
+    public void LoadOrCreate_Throws_WhenSelfSignedCertPathBlank()
+    {
+        TlsOptions options = new() { SelfSignedCertPath = "  " };
+        Assert.Throws<InvalidOperationException>(
+            () => CreateProvider(options, new FakeTimeProvider()).LoadOrCreate());
+    }
+
+    /// <summary>
+    /// Verifies that GenerateAndPersist cleans up the hardened .tmp file when persist fails.
+    /// The failure is induced by setting SelfSignedCertPath to a path whose parent directory
+    /// is an existing regular file, causing Directory.CreateDirectory (or the subsequent write)
+    /// to throw an IOException/UnauthorizedAccessException.
+    /// </summary>
+    [Fact]
+    public void LoadOrCreate_DeletesTempFile_WhenPersistFails()
+    {
+        string outerDir = Directory.CreateTempSubdirectory().FullName;
+        try
+        {
+            // Create a regular file at what would be the parent directory of the cert path.
+            // Any attempt to create that "directory" or write files into it must fail.
+            string fileActingAsDir = Path.Combine(outerDir, "notadir");
+            File.WriteAllText(fileActingAsDir, "block");
+
+            // Point the cert path inside the regular file — Directory.CreateDirectory will
+            // throw because the parent path component is a file, not a directory.
+            string certPath = Path.Combine(fileActingAsDir, "gw.pfx");
+            string expectedTemp = certPath + ".tmp";
+
+            TlsOptions options = new() { SelfSignedCertPath = certPath };
+            Assert.ThrowsAny<Exception>(() => CreateProvider(options, new FakeTimeProvider()).LoadOrCreate());
+
+            // The .tmp file must not be left behind.
+            Assert.False(File.Exists(expectedTemp), $"Leaked temp file: {expectedTemp}");
+        }
+        finally { Directory.Delete(outerDir, recursive: true); }
+    }
+
+    private const string SubjectAltNameOid = "2.5.29.17";
+
+    private static string ReadSubjectAltNames(X509Certificate2 cert)
+        => cert.Extensions
+            .First(e => e.Oid?.Value == SubjectAltNameOid)
+            .Format(false);
+}
@@ -7,6 +7,7 @@

  <ItemGroup>
    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.Extensions.TimeProvider.Testing" Version="10.0.0" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
    <PackageReference Include="xunit" Version="2.9.3" />
    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
Author	SHA1	Message	Date
Joseph Doherty	9bdb899774	fix(clients): inline Go gosec directive and strip IPv6 brackets in Python authority split	2026-06-01 07:57:22 -04:00
Joseph Doherty	e5c704de69	feat(gateway): add machine FQDN to self-signed cert SANs Best-effort resolve the host FQDN via Dns.GetHostEntry and add it as a DNS SAN when it differs (OrdinalIgnoreCase) from the short machine name and "localhost". SocketException / ArgumentException are caught and silently skipped so cert generation remains robust when DNS is absent.	2026-06-01 07:52:48 -04:00
Joseph Doherty	4e520f9c0c	fix(gateway): delete temp cert file on persist failure Wrap the WriteAllBytes/Move/HardenPermissions sequence in a try/catch so that any failure best-effort deletes the hardened .tmp file (which may already hold PFX/private-key bytes) before rethrowing. Add a test that induces a persist failure by pointing SelfSignedCertPath inside a regular file and asserts no .tmp is left on disk.	2026-06-01 07:45:15 -04:00
Joseph Doherty	2eb81379e4	docs: TLS auto-cert and lenient client trust	2026-06-01 07:43:13 -04:00
Joseph Doherty	ddd5721082	fix(gateway): harden self-signed cert persistence and config validation	2026-06-01 07:37:27 -04:00
Joseph Doherty	3775f6bf3b	feat(gateway): supply generated cert as Kestrel HTTPS default	2026-06-01 07:30:26 -04:00
Joseph Doherty	cdfad420bb	fix(client-rust): apply TLS guard to GalaxyClient and add CLI strict flag Extract the TLS-without-CA guard into a shared `build_tls_config` helper in options.rs so both GatewayClient and GalaxyClient use identical logic. GalaxyClient previously had no guard, so TLS-without-CA produced a cryptic tonic handshake failure; it now returns the same actionable InvalidEndpoint error. The guard message notes that a server-name override affects SNI but does not pin trust. Add --require-certificate-validation to ConnectionArgs in the CLI binary. Add a mirror test for GalaxyClient in tests/tls.rs.	2026-06-01 07:28:16 -04:00
Joseph Doherty	330e665f6b	fix(gateway): correct ECDSA key usage and dispose CertificateRequest Drop KeyEncipherment from the self-signed cert's key-usage extension — it is semantically wrong for ECDSA (RSA key-transport only); DigitalSignature alone is correct for TLS 1.3 / ECDHE server certs. CertificateRequest is unchanged (not IDisposable in .NET 10). Test now also asserts MachineName, 127.0.0.1 and IPv6 loopback are present in the SAN extension.	2026-06-01 07:27:15 -04:00
Joseph Doherty	5e01ad9c22	fix(client-dotnet): apply lenient TLS to GalaxyRepositoryClient and enforce hostname on CA-pin Mirror MxGatewayClient's three-branch handler structure in GalaxyRepositoryClient (CA-pin / lenient accept-all / OS trust) so the Galaxy endpoint works against the gateway's self-signed cert under the default lenient posture. Expose an internal CreateHttpHandlerForTests seam for unit testing. Add RemoteCertificateNameMismatch rejection at the top of both CA-pinned callbacks so a pinned-CA connection truly verifies the host. Strengthen existing lenient test to invoke the callback and assert it returns true; add mirrored Galaxy-client handler tests.	2026-06-01 07:24:07 -04:00
Joseph Doherty	77a9108673	feat(gateway): persist/reuse self-signed cert with hardened permissions	2026-06-01 07:23:33 -04:00
Joseph Doherty	192607ab8c	fix(gateway): detect Certificate:Thumbprint and cover more KestrelTlsInspector cases	2026-06-01 07:22:24 -04:00
Joseph Doherty	ba82afe669	fix(client-java): keep Temurin 21 toolchain, auto-provision instead of bumping to 26	2026-06-01 07:20:04 -04:00
Joseph Doherty	fe7d1ce1ec	feat(gateway): validate MxGateway:Tls options	2026-06-01 07:19:22 -04:00
Joseph Doherty	b8a6695612	feat(gateway): generate self-signed ECDSA cert with SANs	2026-06-01 07:18:39 -04:00
Joseph Doherty	6f9188bc8d	test(client-python): update TLS default-channel test for TOFU behavior	2026-06-01 07:17:36 -04:00
Joseph Doherty	a276f46f81	feat(client-java): accept gateway cert by default over TLS	2026-06-01 07:13:45 -04:00
Joseph Doherty	572b268d81	feat(client-rust): accept gateway cert by default over TLS (or documented pin-only fallback)	2026-06-01 07:11:09 -04:00
Joseph Doherty	4c093a64fa	feat(client-python): accept gateway cert by default via TOFU pre-fetch	2026-06-01 07:10:55 -04:00
Joseph Doherty	f47bbaea95	feat(client-dotnet): accept gateway cert by default over TLS	2026-06-01 07:08:55 -04:00
Joseph Doherty	c463b49f46	feat(client-go): accept gateway cert by default over TLS	2026-06-01 07:08:47 -04:00
Joseph Doherty	87f86503ef	feat(gateway): add MxGateway:Tls options block	2026-06-01 07:08:19 -04:00
Joseph Doherty	e912ef960c	feat(gateway): detect HTTPS endpoints missing a certificate	2026-06-01 07:08:12 -04:00
Joseph Doherty	c4e7ddea70	docs: implementation plan for gateway TLS auto-cert and lenient client trust	2026-06-01 07:01:58 -04:00
Joseph Doherty	6bfa4fe884	docs: design for gateway TLS auto-cert and lenient client trust	2026-06-01 06:54:23 -04:00