diff --git a/docs/requirements/Component-ManagementService.md b/docs/requirements/Component-ManagementService.md
index c29d2c88..99450fe2 100644
--- a/docs/requirements/Component-ManagementService.md
+++ b/docs/requirements/Component-ManagementService.md
@@ -134,6 +134,8 @@ Both endpoints honour any site-scope rules attached to the caller's audit role b
 - **CreateDataConnection** / **UpdateDataConnection** / **DeleteDataConnection**: Manage data connection definitions.
 - **AssignDataConnectionToSite** / **UnassignDataConnectionFromSite**: Manage site assignments.
 
+> **Secret elision (arch-review C3).** The `PrimaryConfiguration`/`BackupConfiguration` JSON blobs can embed OPC UA user and certificate passwords, yet List/Get are readable by any authenticated user. All command responses **and** audit `afterState` are projected through a secret-elided shape (`DataConnectionPublicShape`): secret-bearing values inside the config JSON are replaced with a redaction sentinel and a `hasSecrets` flag is surfaced (mirrors the SMTP/SMS `HasCredentials` pattern). **Update** merges the sentinel via `ConfigSecretScrubber.MergeSentinels` — a client that round-trips a scrubbed config keeps the stored secret rather than wiping it.
+
 ### Deployments
 
 - **DeployInstance**: Deploy configuration to a specific instance (includes pre-deployment validation).
diff --git a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
index c773c540..57fd228a 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
@@ -1445,12 +1445,37 @@ public class ManagementActor : ReceiveActor
     // Data Connection handlers
     // ========================================================================
 
+    /// <summary>
+    /// Secret-elided projection of a DataConnection (arch-review C3): the raw
+    /// PrimaryConfiguration/BackupConfiguration JSON can embed OPC UA user and
+    /// certificate passwords, and List/Get are readable by any authenticated user.
+    /// Mirrors <see cref="SmtpConfigPublicShape"/>. The secret-bearing config blobs
+    /// are scrubbed to the redaction sentinel; every non-config field is projected
+    /// verbatim so callers keep the full shape they had before.
+    /// </summary>
+    private static object DataConnectionPublicShape(DataConnection c)
+    {
+        var (primary, s1) = ConfigSecretScrubber.Scrub(c.PrimaryConfiguration);
+        var (backup, s2) = ConfigSecretScrubber.Scrub(c.BackupConfiguration);
+        return new
+        {
+            c.Id,
+            c.SiteId,
+            c.Name,
+            c.Protocol,
+            c.FailoverRetryCount,
+            PrimaryConfiguration = primary,
+            BackupConfiguration = backup,
+            HasSecrets = s1 || s2,
+        };
+    }
+
     private static async Task<object?> HandleListDataConnections(IServiceProvider sp, ListDataConnectionsCommand cmd)
     {
         var repo = sp.GetRequiredService<ISiteRepository>();
         if (cmd.SiteId.HasValue)
-            return await repo.GetDataConnectionsBySiteIdAsync(cmd.SiteId.Value);
-        return await repo.GetAllDataConnectionsAsync();
+            return (await repo.GetDataConnectionsBySiteIdAsync(cmd.SiteId.Value)).Select(DataConnectionPublicShape).ToList();
+        return (await repo.GetAllDataConnectionsAsync()).Select(DataConnectionPublicShape).ToList();
     }
 
     private static async Task<object?> HandleGetDataConnection(IServiceProvider sp, GetDataConnectionCommand cmd, AuthenticatedUser user)
@@ -1459,7 +1484,7 @@ public class ManagementActor : ReceiveActor
         var conn = await repo.GetDataConnectionByIdAsync(cmd.DataConnectionId);
         if (conn != null)
             EnforceSiteScope(user, conn.SiteId);
-        return conn;
+        return conn is null ? null : DataConnectionPublicShape(conn);
     }
 
     private static async Task<object?> HandleCreateDataConnection(IServiceProvider sp, CreateDataConnectionCommand cmd, string user)
@@ -1473,8 +1498,9 @@ public class ManagementActor : ReceiveActor
         };
         await repo.AddDataConnectionAsync(conn);
         await repo.SaveChangesAsync();
-        await AuditAsync(sp, user, "Create", "DataConnection", conn.Id.ToString(), conn.Name, conn);
-        return conn;
+        var publicShape = DataConnectionPublicShape(conn);
+        await AuditAsync(sp, user, "Create", "DataConnection", conn.Id.ToString(), conn.Name, publicShape);
+        return publicShape;
     }
 
     private static async Task<object?> HandleUpdateDataConnection(IServiceProvider sp, UpdateDataConnectionCommand cmd, string user)
@@ -1484,13 +1510,17 @@ public class ManagementActor : ReceiveActor
             ?? throw new ManagementCommandException($"DataConnection with ID {cmd.DataConnectionId} not found.");
         conn.Name = cmd.Name;
         conn.Protocol = cmd.Protocol;
-        conn.PrimaryConfiguration = cmd.PrimaryConfiguration;
-        conn.BackupConfiguration = cmd.BackupConfiguration;
+        // Sentinel-preserving merge: a client that round-trips a scrubbed config
+        // (secrets replaced with the redaction sentinel) keeps the stored secret
+        // rather than wiping it. Non-sentinel values overwrite as usual.
+        conn.PrimaryConfiguration = ConfigSecretScrubber.MergeSentinels(cmd.PrimaryConfiguration, conn.PrimaryConfiguration);
+        conn.BackupConfiguration = ConfigSecretScrubber.MergeSentinels(cmd.BackupConfiguration, conn.BackupConfiguration);
         conn.FailoverRetryCount = cmd.FailoverRetryCount;
         await repo.UpdateDataConnectionAsync(conn);
         await repo.SaveChangesAsync();
-        await AuditAsync(sp, user, "Update", "DataConnection", conn.Id.ToString(), conn.Name, conn);
-        return conn;
+        var publicShape = DataConnectionPublicShape(conn);
+        await AuditAsync(sp, user, "Update", "DataConnection", conn.Id.ToString(), conn.Name, publicShape);
+        return publicShape;
     }
 
     private static async Task<object?> HandleDeleteDataConnection(IServiceProvider sp, DeleteDataConnectionCommand cmd, string user)
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecretProjectionTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecretProjectionTests.cs
new file mode 100644
index 00000000..e474f1dc
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecretProjectionTests.cs
@@ -0,0 +1,134 @@
+using Akka.Actor;
+using Akka.TestKit.Xunit2;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+using NSubstitute;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.ExternalSystems;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Sites;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Management;
+using ZB.MOM.WW.ScadaBridge.Commons.Types;
+
+namespace ZB.MOM.WW.ScadaBridge.ManagementService.Tests;
+
+/// <summary>
+/// Secret-elision projection tests (arch-review C3): connection-config secrets
+/// (OPC UA passwords, external-system auth config, DB connection strings) must
+/// never leave the management boundary via List/Get responses or audit afterState,
+/// and sentinel/omitted round-trips through Update must preserve the stored secret.
+/// </summary>
+public class SecretProjectionTests : TestKit, IDisposable
+{
+    private readonly IAuditService _auditService;
+    private readonly ServiceCollection _services;
+
+    public SecretProjectionTests()
+    {
+        _auditService = Substitute.For<IAuditService>();
+        _services = new ServiceCollection();
+        _services.AddScoped(_ => _auditService);
+    }
+
+    private IActorRef CreateActor()
+    {
+        var sp = _services.BuildServiceProvider();
+        return Sys.ActorOf(Props.Create(() => new ManagementActor(
+            sp, NullLogger<ManagementActor>.Instance)));
+    }
+
+    private static ManagementEnvelope Envelope(object command, params string[] roles) =>
+        new(new AuthenticatedUser("testuser", "Test User", roles, Array.Empty<string>()),
+            command, Guid.NewGuid().ToString("N"));
+
+    void IDisposable.Dispose() => Shutdown();
+
+    // ========================================================================
+    // Task 7 — DataConnection config secret elision
+    // ========================================================================
+
+    private ISiteRepository AddSiteRepoWithConnection(DataConnection conn)
+    {
+        var siteRepo = Substitute.For<ISiteRepository>();
+        siteRepo.GetDataConnectionByIdAsync(conn.Id, Arg.Any<CancellationToken>()).Returns(conn);
+        siteRepo.GetAllDataConnectionsAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<DataConnection> { conn });
+        _services.AddScoped(_ => siteRepo);
+        return siteRepo;
+    }
+
+    [Fact]
+    public void GetDataConnection_ResponseOmitsPasswords()
+    {
+        AddSiteRepoWithConnection(new DataConnection("c1", "OpcUa", 1)
+        {
+            Id = 1,
+            PrimaryConfiguration = """{"Endpoint":"opc.tcp://x","UserIdentity":{"Password":"opc-secret"}}""",
+        });
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new GetDataConnectionCommand(1), "Viewer"));
+
+        var resp = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.DoesNotContain("opc-secret", resp.JsonData);
+        Assert.Contains("hasSecrets", resp.JsonData); // camelCase serializer
+    }
+
+    [Fact]
+    public void ListDataConnections_ResponseOmitsPasswords()
+    {
+        AddSiteRepoWithConnection(new DataConnection("c1", "OpcUa", 1)
+        {
+            Id = 1,
+            PrimaryConfiguration = """{"UserIdentity":{"Password":"opc-secret"}}""",
+        });
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new ListDataConnectionsCommand(null), "Viewer"));
+
+        var resp = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.DoesNotContain("opc-secret", resp.JsonData);
+        Assert.Contains("hasSecrets", resp.JsonData);
+    }
+
+    [Fact]
+    public async Task UpdateDataConnection_SentinelConfig_PreservesStoredSecret()
+    {
+        var stored = new DataConnection("c1", "OpcUa", 1)
+        {
+            Id = 1,
+            PrimaryConfiguration = """{"Endpoint":"opc.tcp://old","UserIdentity":{"Password":"opc-secret"}}""",
+        };
+        var siteRepo = AddSiteRepoWithConnection(stored);
+
+        var actor = CreateActor();
+        var incoming = """{"Endpoint":"opc.tcp://new","UserIdentity":{"Password":"***REDACTED***"}}""";
+        actor.Tell(Envelope(new UpdateDataConnectionCommand(1, "c1", "OpcUa", incoming, null, 3), "Designer"));
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+
+        await siteRepo.Received(1).UpdateDataConnectionAsync(
+            Arg.Is<DataConnection>(c => c.PrimaryConfiguration!.Contains("opc-secret")
+                                     && !c.PrimaryConfiguration.Contains("***REDACTED***")),
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public void UpdateDataConnection_AuditAfterState_OmitsPasswords()
+    {
+        var stored = new DataConnection("c1", "OpcUa", 1)
+        {
+            Id = 1,
+            PrimaryConfiguration = """{"Password":"opc-secret"}""",
+        };
+        AddSiteRepoWithConnection(stored);
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new UpdateDataConnectionCommand(1, "c1", "OpcUa", """{"Password":"newpw"}""", null, 3), "Designer"));
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+
+        _auditService.Received().LogAsync(Arg.Any<string>(), "Update", "DataConnection",
+            Arg.Any<string>(), Arg.Any<string>(),
+            Arg.Is<object>(o => !ManagementActor.SerializeResult(o).Contains("newpw")),
+            Arg.Any<CancellationToken>());
+    }
+}