Stack overflow (0xc00000fd) in RPCRT4.dll when using Bot Media SDK with .NET 9.0

[mprachar]

Description

When using the Microsoft Bot Media SDK (Microsoft.Skype.Bots.Media version 1.31.0.225-preview) with .NET 9.0, the application crashes with a stack overflow (STATUS_STACK_OVERFLOW / 0xc00000fd) in RPCRT4.dll shortly after successfully joining a Teams meeting.

Environment

• OS: Windows Server 2022 Datacenter (Azure VM)
• .NET Version: .NET 9.0
• SDK Versions:
  • Microsoft.Skype.Bots.Media: 1.31.0.225-preview
  • Microsoft.Graph.Communications.Calls.Media: 1.2.0.15690
  • Microsoft.Graph.Communications.Client: 1.2.0.15690

Symptoms

1. Bot successfully authenticates and joins the Teams meeting
2. Media session is established (call state transitions to Established)
3. ~5-10 seconds after joining, the process crashes with:
   • Exception Code: 0xc00000fd (STATUS_STACK_OVERFLOW)
   • Faulting Module: RPCRT4.dll
   • Faulting Application: VoiceRail.Voice.exe

Investigation Findings

We extensively tested various hypotheses:

1. Disabled ALL event handlers (AudioMediaReceived, AudioSendStatusChanged, Participants.OnUpdated) - still crashed
2. Offloaded event handlers to ThreadPool - still crashed
3. Deferred event subscription from constructor - still crashed
4. Tested SDK version 1.31.0.225-preview (minimum required by Calls.Media) - still crashed

The crash occurs INSIDE the SDK's native code, not in our managed code.

Root Cause

After extensive research, we identified this as a .NET 9.0 Control Flow Guard (CFG) compatibility issue:

• .NET 9.0 introduced stricter CFG/CET (Control-flow Enforcement Technology) enforcement
• The Bot Media SDK's native DLLs make RPC callbacks into managed code
• These callbacks fail CFG stack validation, causing the stack overflow in RPCRT4.dll
• Related issue: Stack overflow when calling LoadLibrary on .NET 9.0, but not on .NET 8.0 dotnet/runtime#119067

Workaround

Adding <CETCompat>false</CETCompat> to the project file resolves the issue:

<PropertyGroup>
  <TargetFramework>net9.0</TargetFramework>
  <!-- Disable CET/CFG compatibility for Bot Media SDK native interop -->
  <CETCompat>false</CETCompat>
</PropertyGroup>

Request

1. Documentation: Please document the .NET 9.0 compatibility issue and the CETCompat workaround
2. SDK Update: Consider updating the Bot Media SDK to be compatible with .NET 9.0's CFG enforcement
3. Official Support: Clarify if .NET 9.0 is officially supported or if .NET 8.0 LTS should be used

Reproduction Steps

1. Create a new .NET 9.0 project targeting win-x64
2. Add Microsoft.Skype.Bots.Media 1.31.0.225-preview
3. Implement a basic Teams meeting bot following the samples
4. Deploy to Windows Server 2022
5. Join a Teams meeting
6. Observe crash ~5-10 seconds after joining

Additional Context

• The same code works correctly when <CETCompat>false</CETCompat> is added
• We have not tested with .NET 8.0 (downgrade would also likely resolve the issue)
• All Windows Media Foundation components and VC++ Redistributables are installed

[ssulzer]

@mprachar
Please try version 1.32 of the media SDK, it does have a fix to help with .NET 9.0 support.
https://www.nuget.org/packages/Microsoft.Skype.Bots.Media/1.32.0.70-preview

However, we were not aware of the Control-flow Enforcement issue and likely you will need to use the workaround you identified. Thanks for reporting this.