diff --git a/src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Enums/NodePermissions.cs b/src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Enums/NodePermissions.cs
index 44a30628..87c1937d 100644
--- a/src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Enums/NodePermissions.cs
+++ b/src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Enums/NodePermissions.cs
@@ -29,6 +29,11 @@ public enum NodePermissions : int
     // OPC UA Part 4 §5.11
     MethodCall = 1 << 11,
 
+    // OPC UA HistoryUpdate (annotation / insert / delete) — separate from HistoryRead so a
+    // read-only grant cannot authorize historian writes. Not included in any composite bundle
+    // until the HistoryUpdate service surface is implemented.
+    HistoryUpdate = 1 << 12,
+
     // Bundles (one-click grants in Admin UI)
     ReadOnly = Browse | Read | Subscribe | HistoryRead | AlarmRead,
     Operator = ReadOnly | WriteOperate | AlarmAcknowledge | AlarmConfirm,
diff --git a/src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/TriePermissionEvaluator.cs b/src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/TriePermissionEvaluator.cs
index a92608cc..a5881c3a 100644
--- a/src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/TriePermissionEvaluator.cs
+++ b/src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/TriePermissionEvaluator.cs
@@ -83,7 +83,7 @@ public sealed class TriePermissionEvaluator : IPermissionEvaluator
         OpcUaOperation.WriteTune            => NodePermissions.WriteTune,
         OpcUaOperation.WriteConfigure       => NodePermissions.WriteConfigure,
         OpcUaOperation.HistoryRead          => NodePermissions.HistoryRead,
-        OpcUaOperation.HistoryUpdate        => NodePermissions.HistoryRead, // HistoryUpdate bit not yet in NodePermissions; TODO Stream C follow-up
+        OpcUaOperation.HistoryUpdate        => NodePermissions.HistoryUpdate,
         OpcUaOperation.CreateMonitoredItems => NodePermissions.Subscribe,
         OpcUaOperation.TransferSubscriptions=> NodePermissions.Subscribe,
         OpcUaOperation.Call                 => NodePermissions.MethodCall,
diff --git a/tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/Authorization/TriePermissionEvaluatorTests.cs b/tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/Authorization/TriePermissionEvaluatorTests.cs
index 5309997c..4d8b5c4e 100644
--- a/tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/Authorization/TriePermissionEvaluatorTests.cs
+++ b/tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/Authorization/TriePermissionEvaluatorTests.cs
@@ -191,6 +191,38 @@ public sealed class TriePermissionEvaluatorTests
             "a session bound to a generation absent from the cache must fail closed");
     }
 
+    /// <summary>Verifies that a HistoryRead-only grant does NOT authorize HistoryUpdate.</summary>
+    [Fact]
+    public void HistoryRead_grant_does_not_authorize_HistoryUpdate()
+    {
+        // Before the fix HistoryUpdate was mapped to the HistoryRead bit, so a read-only grant
+        // would wrongly authorise a write operation.
+        var evaluator = MakeEvaluator([Row("cn=ops", NodeAclScopeKind.Cluster, null, NodePermissions.HistoryRead)]);
+
+        var decision = evaluator.Authorize(Session(["cn=ops"]), OpcUaOperation.HistoryUpdate, Scope());
+
+        decision.IsAllowed.ShouldBeFalse("HistoryRead grant must NOT imply HistoryUpdate");
+    }
+
+    /// <summary>Verifies that a HistoryUpdate grant authorizes HistoryUpdate.</summary>
+    [Fact]
+    public void HistoryUpdate_grant_authorizes_HistoryUpdate()
+    {
+        var evaluator = MakeEvaluator([Row("cn=ops", NodeAclScopeKind.Cluster, null, NodePermissions.HistoryUpdate)]);
+
+        var decision = evaluator.Authorize(Session(["cn=ops"]), OpcUaOperation.HistoryUpdate, Scope());
+
+        decision.IsAllowed.ShouldBeTrue("HistoryUpdate grant must authorize HistoryUpdate operation");
+    }
+
+    /// <summary>Verifies that the HistoryUpdate bit is 1&lt;&lt;12 and does not collide with MethodCall.</summary>
+    [Fact]
+    public void HistoryUpdate_bit_value_and_no_collision()
+    {
+        ((int)NodePermissions.HistoryUpdate).ShouldBe(1 << 12);
+        (NodePermissions.HistoryUpdate & NodePermissions.MethodCall).ShouldBe(NodePermissions.None);
+    }
+
     /// <summary>Verifies that the operation-to-permission mapping is total.</summary>
     [Fact]
     public void OperationToPermission_Mapping_IsTotal()