diff --git a/dotnet/Directory.Packages.props b/dotnet/Directory.Packages.props
index 4469eea47e..f3750bde87 100644
--- a/dotnet/Directory.Packages.props
+++ b/dotnet/Directory.Packages.props
@@ -13,6 +13,7 @@
     <PackageVersion Include="CommunityToolkit.Aspire.Hosting.Dapr" Version="9.9.0" />
     <PackageVersion Include="CommunityToolkit.Aspire.Hosting.NodeJS.Extensions" Version="9.9.0" />
     <PackageVersion Include="Aspire.Hosting.Azure.Search" Version="13.0.0" />
+    <PackageVersion Include="MessagePack" Version="2.5.301" /><!-- Vulnerability fix: GHSA-hv8m-jj95-wg3x; transitively pinned via Aspire.Hosting.AppHost -->
     <PackageVersion Include="AWSSDK.BedrockAgent" Version="4.0.7.5" />
     <PackageVersion Include="AWSSDK.BedrockAgentRuntime" Version="4.0.8.5" />
     <PackageVersion Include="AWSSDK.BedrockRuntime" Version="4.0.14.5" />
diff --git a/dotnet/samples/Demos/AgentFrameworkWithAspire/ChatWithAgent.AppHost/ChatWithAgent.AppHost.csproj b/dotnet/samples/Demos/AgentFrameworkWithAspire/ChatWithAgent.AppHost/ChatWithAgent.AppHost.csproj
index 0539702b56..5d8a2229c5 100644
--- a/dotnet/samples/Demos/AgentFrameworkWithAspire/ChatWithAgent.AppHost/ChatWithAgent.AppHost.csproj
+++ b/dotnet/samples/Demos/AgentFrameworkWithAspire/ChatWithAgent.AppHost/ChatWithAgent.AppHost.csproj
@@ -20,6 +20,7 @@
 
   <ItemGroup>
     <PackageReference Include="Aspire.Hosting.AppHost" />
+    <PackageReference Include="MessagePack" /><!-- Pin to patched version; overrides vulnerable transitive 2.5.192 from Aspire.Hosting.AppHost (GHSA-hv8m-jj95-wg3x) -->
     <PackageReference Include="Aspire.Hosting.Azure.CognitiveServices" />
     <PackageReference Include="Aspire.Hosting.Azure.Search" />
   </ItemGroup>