diff --git a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
index d36d927f..98ec8964 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
@@ -1045,6 +1045,35 @@ public class ManagementActor : ReceiveActor
         await EmitSecuredWriteAuditAsync(
             sp, AuditKind.SecuredWriteApprove, AuditStatus.Submitted, row, actor: user.Username);
 
+        // D2 / T14 TOCTOU guard: re-assert the MxGateway protocol AT EXECUTE, not only at
+        // submit. The connection named on the row may have been reconfigured/recreated as a
+        // non-MxGateway (e.g. OPC UA) connection between submit and approval; relaying then
+        // would execute the secured write against a non-MxGateway adapter, violating the
+        // feature's core safety invariant. Re-load the connection with the same lookup
+        // submit uses (resolve site -> its data connections -> match by name) and fail the
+        // row deterministically (mirrors the decode-error containment below) if it is
+        // missing or no longer MxGateway — WITHOUT relaying.
+        var siteRepo = sp.GetRequiredService<ISiteRepository>();
+        var execSite = await siteRepo.GetSiteByIdentifierAsync(row.SiteId);
+        var execConn = execSite is null
+            ? null
+            : (await siteRepo.GetDataConnectionsBySiteIdAsync(execSite.Id))
+                .FirstOrDefault(c => string.Equals(c.Name, row.ConnectionName, StringComparison.Ordinal));
+        if (execConn is null ||
+            !string.Equals(execConn.Protocol, "MxGateway", StringComparison.OrdinalIgnoreCase))
+        {
+            row.Status = "Failed";
+            row.ExecutedAtUtc = DateTime.UtcNow;
+            row.ExecutionError = execConn is null
+                ? $"connection '{row.ConnectionName}' not found"
+                : $"connection '{row.ConnectionName}' is no longer an MxGateway connection";
+            await repo.UpdateAsync(row);
+            await EmitSecuredWriteAuditAsync(
+                sp, AuditKind.SecuredWriteExecute, AuditStatus.Failed, row,
+                actor: user.Username, errorMessage: row.ExecutionError);
+            return ToSecuredWriteDto(row);
+        }
+
         // Validate the value type BEFORE attempting the relay. An unknown type can
         // never be decoded/written, so fail the row deterministically rather than
         // leaving it stuck Approved. (Addresses the C2 reviewer's deferred
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecuredWriteHandlerTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecuredWriteHandlerTests.cs
index bf0a66c0..7ee7deb5 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecuredWriteHandlerTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/SecuredWriteHandlerTests.cs
@@ -225,6 +225,12 @@ public class SecuredWriteHandlerTests : TestKit, IDisposable
             SubmittedAtUtc = DateTime.UtcNow
         };
         _securedWriteRepo.GetAsync(id, Arg.Any<CancellationToken>()).Returns(row);
+        // The approve handler re-asserts the MxGateway protocol AT EXECUTE (D2 / T14
+        // TOCTOU guard), so the connection named on the row must still resolve to an
+        // MxGateway connection at approve-time for the relay to proceed. Seed it here
+        // so every approve test that should relay sees a valid MxGateway target; tests
+        // exercising the reconfigured/missing-connection case re-stub this lookup.
+        SeedSiteWithConnection(1, "SITE1", "Mx1", "MxGateway");
         return row;
     }
 
@@ -501,6 +507,75 @@ public class SecuredWriteHandlerTests : TestKit, IDisposable
         Assert.Equal("unknown value type", updated.ExecutionError);
     }
 
+    [Fact]
+    public void Approve_WhenConnectionNoLongerMxGateway_FlipsStatusToFailed_NoRelay()
+    {
+        // D2 / T14 TOCTOU guard: the MxGateway protocol is re-asserted AT EXECUTE, not
+        // only at submit. The named connection was reconfigured/recreated as a
+        // non-MxGateway (OPC UA) connection between submit and approval; relaying then
+        // would execute the secured write against a non-MxGateway adapter, violating the
+        // feature's core safety invariant. The row must fail deterministically with the
+        // protocol error and NO relay — even though the CAS already won.
+        SeedPendingWrite(7, operatorUser: "alice");
+        ArmCasSuccess(7);
+        // Re-stub the approve-time connection lookup to a non-MxGateway protocol,
+        // overriding the MxGateway seed inside SeedPendingWrite.
+        SeedSiteWithConnection(1, "SITE1", "Mx1", "OpcUa");
+        var captured = CaptureAuditEvents();
+
+        PendingSecuredWrite? updated = null;
+        _securedWriteRepo
+            .When(r => r.UpdateAsync(Arg.Any<PendingSecuredWrite>(), Arg.Any<CancellationToken>()))
+            .Do(ci => updated = ci.Arg<PendingSecuredWrite>());
+
+        var actor = CreateActor();
+        var envelope = Envelope(new ApproveSecuredWriteCommand(7, "approved"), "bob", "Verifier");
+
+        actor.Tell(envelope);
+
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        // No relay — the connection is no longer an MxGateway target.
+        Assert.Equal(0, _comms.CallCount);
+        Assert.NotNull(updated);
+        Assert.Equal("Failed", updated!.Status);
+        Assert.NotNull(updated.ExecutedAtUtc);
+        Assert.Contains("no longer an MxGateway connection", updated.ExecutionError);
+
+        // The Execute audit row records the failure (canonical Failure outcome).
+        WaitForAuditRows(captured, 2);
+        var execute = SingleOfKind(captured, AuditKind.SecuredWriteExecute);
+        Assert.Equal(ZB.MOM.WW.Audit.AuditOutcome.Failure, execute.Outcome);
+    }
+
+    [Fact]
+    public void Approve_WhenConnectionDeleted_FlipsStatusToFailed_NotFound_NoRelay()
+    {
+        // TOCTOU companion: the connection named on the row was deleted between submit
+        // and approval. The re-load at execute returns no match → fail with "not found"
+        // and no relay.
+        SeedPendingWrite(7, operatorUser: "alice");
+        ArmCasSuccess(7);
+        // Re-stub the approve-time lookup so the site resolves but exposes no connections.
+        _siteRepo.GetDataConnectionsBySiteIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<DataConnection>());
+
+        PendingSecuredWrite? updated = null;
+        _securedWriteRepo
+            .When(r => r.UpdateAsync(Arg.Any<PendingSecuredWrite>(), Arg.Any<CancellationToken>()))
+            .Do(ci => updated = ci.Arg<PendingSecuredWrite>());
+
+        var actor = CreateActor();
+        var envelope = Envelope(new ApproveSecuredWriteCommand(7, "approved"), "bob", "Verifier");
+
+        actor.Tell(envelope);
+
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.Equal(0, _comms.CallCount);
+        Assert.NotNull(updated);
+        Assert.Equal("Failed", updated!.Status);
+        Assert.Contains("not found", updated.ExecutionError);
+    }
+
     // ------------------------------------------------------------------------
     // Audit emission (T14b — SecuredWrite AuditLog rows)
     // ------------------------------------------------------------------------