From bca2ded8eb8d8467aae96b4277da34aa894e84bd Mon Sep 17 00:00:00 2001
From: Suhas Holla Karkada Chandrashekar
 <51073384+suhashollakc@users.noreply.github.com>
Date: Sat, 13 Jun 2026 12:04:42 -0400
Subject: [PATCH] Fix MQTT v5 topic alias forwarding (#2250)

---
 Source/MQTTnet.Server/Internal/MqttConnectedClient.cs | 4 +++-
 Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs           | 4 ++--
 Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs      | 5 ++++-
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/Source/MQTTnet.Server/Internal/MqttConnectedClient.cs b/Source/MQTTnet.Server/Internal/MqttConnectedClient.cs
index 48e6bd2c..5fd70d48 100644
--- a/Source/MQTTnet.Server/Internal/MqttConnectedClient.cs
+++ b/Source/MQTTnet.Server/Internal/MqttConnectedClient.cs
@@ -218,6 +218,8 @@ public sealed class MqttConnectedClient : IDisposable
         HandleTopicAlias(publishPacket);
 
         var applicationMessage = MqttApplicationMessageFactory.Create(publishPacket);
+        // Topic aliases are scoped to this network connection and must not be forwarded to other clients.
+        applicationMessage.TopicAlias = 0;
 
         var dispatchApplicationMessageResult =
             await _sessionsManager.DispatchApplicationMessage(Id, UserName, Session.Items, applicationMessage, cancellationToken).ConfigureAwait(false);
@@ -579,4 +581,4 @@ public sealed class MqttConnectedClient : IDisposable
             _logger.Warning(exception, "Client '{0}': Error while sending DISCONNECT packet (ReasonCode = {1})", Id, options.ReasonCode);
         }
     }
-}
\ No newline at end of file
+}
diff --git a/Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs b/Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs
index f68323ba..7ecfda35 100644
--- a/Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs
+++ b/Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs
@@ -114,7 +114,7 @@ public sealed class Client_Tests : BaseTestClass
 
         Assert.IsNotNull(receivedMessage);
         Assert.AreEqual(applicationMessage.Topic, receivedMessage.Topic);
-        Assert.AreEqual(applicationMessage.TopicAlias, receivedMessage.TopicAlias);
+        Assert.AreEqual(0, receivedMessage.TopicAlias);
         Assert.AreEqual(applicationMessage.ContentType, receivedMessage.ContentType);
         Assert.AreEqual(applicationMessage.ResponseTopic, receivedMessage.ResponseTopic);
         Assert.AreEqual(applicationMessage.MessageExpiryInterval, receivedMessage.MessageExpiryInterval);
@@ -344,4 +344,4 @@ public sealed class Client_Tests : BaseTestClass
         memoryStream.Position = 0;
         return memoryStream;
     }
-}
\ No newline at end of file
+}
diff --git a/Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs b/Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs
index 86469640..51635238 100644
--- a/Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs
+++ b/Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs
@@ -34,6 +34,7 @@ public sealed class Topic_Alias_Tests : BaseTestClass
         await testEnvironment.StartServer();
 
         var receivedTopics = new List<string>();
+        var receivedTopicAliases = new List<ushort>();
 
         var c1 = await testEnvironment.ConnectClient(options => options.WithProtocolVersion(MqttProtocolVersion.V500));
         c1.ApplicationMessageReceivedAsync += e =>
@@ -41,6 +42,7 @@ public sealed class Topic_Alias_Tests : BaseTestClass
             lock (receivedTopics)
             {
                 receivedTopics.Add(e.ApplicationMessage.Topic);
+                receivedTopicAliases.Add(e.ApplicationMessage.TopicAlias);
             }
 
             return CompletedTask.Instance;
@@ -68,5 +70,6 @@ public sealed class Topic_Alias_Tests : BaseTestClass
         Assert.HasCount(3, receivedTopics);
         CollectionAssert.AllItemsAreNotNull(receivedTopics);
         Assert.IsTrue(receivedTopics.All(t => t.Equals("this_is_the_topic", System.StringComparison.Ordinal)));
+        Assert.IsTrue(receivedTopicAliases.All(a => a == 0));
     }
-}
\ No newline at end of file
+}