diff --git a/docs/requirements/Component-ManagementService.md b/docs/requirements/Component-ManagementService.md
index 99450fe2..3931eecc 100644
--- a/docs/requirements/Component-ManagementService.md
+++ b/docs/requirements/Component-ManagementService.md
@@ -156,6 +156,8 @@ The two-person authorization workflow for writes through the MxAccess Gateway. B
 - **ListExternalSystems** / **GetExternalSystem**: Query external system definitions.
 - **CreateExternalSystem** / **UpdateExternalSystem** / **DeleteExternalSystem**: Manage external system definitions.
 
+> **Secret elision (arch-review C3).** The `AuthConfiguration` blob holds API keys / Basic credentials. All command responses **and** audit `afterState` are projected through `ExternalSystemPublicShape`, which drops `AuthConfiguration` entirely and surfaces only a `hasAuthConfiguration` presence flag. **Update** is preserve-if-null: an omitted (`null`) `AuthConfiguration` leaves the stored secret intact (mirrors the SMTP/SMS credential rule).
+
 ### Notifications
 
 - **ListNotificationLists** / **GetNotificationList**: Query notification lists.
diff --git a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
index 57fd228a..239c6ed2 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
@@ -1661,16 +1661,36 @@ public class ManagementActor : ReceiveActor
     // External System handlers
     // ========================================================================
 
+    /// <summary>
+    /// Secret-elided ExternalSystemDefinition projection (arch-review C3): the
+    /// opaque <c>AuthConfiguration</c> blob holds API keys / Basic credentials and
+    /// must never leave this boundary via responses or audit afterState. Presence
+    /// is surfaced as <c>HasAuthConfiguration</c>; every non-secret field is
+    /// projected verbatim. Mirrors <see cref="SmtpConfigPublicShape"/>.
+    /// </summary>
+    private static object ExternalSystemPublicShape(ExternalSystemDefinition d) => new
+    {
+        d.Id,
+        d.Name,
+        d.EndpointUrl,
+        d.AuthType,
+        d.MaxRetries,
+        d.RetryDelay,
+        d.TimeoutSeconds,
+        HasAuthConfiguration = !string.IsNullOrEmpty(d.AuthConfiguration),
+    };
+
     private static async Task<object?> HandleListExternalSystems(IServiceProvider sp)
     {
         var repo = sp.GetRequiredService<IExternalSystemRepository>();
-        return await repo.GetAllExternalSystemsAsync();
+        return (await repo.GetAllExternalSystemsAsync()).Select(ExternalSystemPublicShape).ToList();
     }
 
     private static async Task<object?> HandleGetExternalSystem(IServiceProvider sp, GetExternalSystemCommand cmd)
     {
         var repo = sp.GetRequiredService<IExternalSystemRepository>();
-        return await repo.GetExternalSystemByIdAsync(cmd.ExternalSystemId);
+        var def = await repo.GetExternalSystemByIdAsync(cmd.ExternalSystemId);
+        return def is null ? null : ExternalSystemPublicShape(def);
     }
 
     private static async Task<object?> HandleCreateExternalSystem(IServiceProvider sp, CreateExternalSystemCommand cmd, string user)
@@ -1683,8 +1703,9 @@ public class ManagementActor : ReceiveActor
         };
         await repo.AddExternalSystemAsync(def);
         await repo.SaveChangesAsync();
-        await AuditAsync(sp, user, "Create", "ExternalSystem", def.Id.ToString(), def.Name, def);
-        return def;
+        var publicShape = ExternalSystemPublicShape(def);
+        await AuditAsync(sp, user, "Create", "ExternalSystem", def.Id.ToString(), def.Name, publicShape);
+        return publicShape;
     }
 
     private static async Task<object?> HandleUpdateExternalSystem(IServiceProvider sp, UpdateExternalSystemCommand cmd, string user)
@@ -1695,12 +1716,15 @@ public class ManagementActor : ReceiveActor
         def.Name = cmd.Name;
         def.EndpointUrl = cmd.EndpointUrl;
         def.AuthType = cmd.AuthType;
-        def.AuthConfiguration = cmd.AuthConfiguration;
+        // Preserve-if-null: an update that omits AuthConfiguration leaves the stored
+        // secret intact (mirrors the SMTP/SMS credential preserve-if-null rule).
+        if (cmd.AuthConfiguration is not null) def.AuthConfiguration = cmd.AuthConfiguration;
         if (cmd.TimeoutSeconds is { } ts) def.TimeoutSeconds = ts;
         await repo.UpdateExternalSystemAsync(def);
         await repo.SaveChangesAsync();
-        await AuditAsync(sp, user, "Update", "ExternalSystem", def.Id.ToString(), def.Name, def);
-        return def;
+        var publicShape = ExternalSystemPublicShape(def);
+        await AuditAsync(sp, user, "Update", "ExternalSystem", def.Id.ToString(), def.Name, publicShape);
+        return publicShape;
     }
 
     private static async Task<object?> HandleDeleteExternalSystem(IServiceProvider sp, DeleteExternalSystemCommand 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
index e474f1dc..1b276fae 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecretProjectionTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecretProjectionTests.cs
@@ -131,4 +131,91 @@ public class SecretProjectionTests : TestKit, IDisposable
             Arg.Is<object>(o => !ManagementActor.SerializeResult(o).Contains("newpw")),
             Arg.Any<CancellationToken>());
     }
+
+    // ========================================================================
+    // Task 8 — ExternalSystem AuthConfiguration elision
+    // ========================================================================
+
+    private IExternalSystemRepository AddExternalSystemRepo(ExternalSystemDefinition def)
+    {
+        var repo = Substitute.For<IExternalSystemRepository>();
+        repo.GetExternalSystemByIdAsync(def.Id, Arg.Any<CancellationToken>()).Returns(def);
+        repo.GetAllExternalSystemsAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<ExternalSystemDefinition> { def });
+        _services.AddScoped(_ => repo);
+        return repo;
+    }
+
+    [Fact]
+    public void GetExternalSystem_ResponseOmitsAuthConfiguration()
+    {
+        AddExternalSystemRepo(new ExternalSystemDefinition("es", "https://x", "ApiKey")
+        {
+            Id = 1,
+            AuthConfiguration = """{"apiKey":"es-secret"}""",
+        });
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new GetExternalSystemCommand(1), "Viewer"));
+
+        var resp = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.DoesNotContain("es-secret", resp.JsonData);
+        Assert.Contains("hasAuthConfiguration", resp.JsonData);
+    }
+
+    [Fact]
+    public void ListExternalSystems_ResponseOmitsAuthConfiguration()
+    {
+        AddExternalSystemRepo(new ExternalSystemDefinition("es", "https://x", "ApiKey")
+        {
+            Id = 1,
+            AuthConfiguration = """{"apiKey":"es-secret"}""",
+        });
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new ListExternalSystemsCommand(), "Viewer"));
+
+        var resp = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.DoesNotContain("es-secret", resp.JsonData);
+        Assert.Contains("hasAuthConfiguration", resp.JsonData);
+    }
+
+    [Fact]
+    public async Task UpdateExternalSystem_NullAuthConfiguration_PreservesStoredSecret()
+    {
+        var stored = new ExternalSystemDefinition("es", "https://x", "ApiKey")
+        {
+            Id = 1,
+            AuthConfiguration = """{"apiKey":"es-secret"}""",
+        };
+        var repo = AddExternalSystemRepo(stored);
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new UpdateExternalSystemCommand(1, "es", "https://y", "ApiKey", null), "Designer"));
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+
+        await repo.Received(1).UpdateExternalSystemAsync(
+            Arg.Is<ExternalSystemDefinition>(d => d.AuthConfiguration == """{"apiKey":"es-secret"}"""),
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public void UpdateExternalSystem_AuditAfterState_OmitsAuthConfiguration()
+    {
+        var stored = new ExternalSystemDefinition("es", "https://x", "ApiKey")
+        {
+            Id = 1,
+            AuthConfiguration = """{"apiKey":"es-secret"}""",
+        };
+        AddExternalSystemRepo(stored);
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new UpdateExternalSystemCommand(1, "es", "https://y", "ApiKey", """{"apiKey":"newsecret"}"""), "Designer"));
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+
+        _auditService.Received().LogAsync(Arg.Any<string>(), "Update", "ExternalSystem",
+            Arg.Any<string>(), Arg.Any<string>(),
+            Arg.Is<object>(o => !ManagementActor.SerializeResult(o).Contains("newsecret")),
+            Arg.Any<CancellationToken>());
+    }
 }