MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.303 and 3.1.9, deserializing attacker-controlled MessagePack data as System.Object through PrimitiveObjectFormatter can cause excessive memory allocation. The formatter allocates an object[] from each declared array length before the configured object-graph depth limit rejects the input, while nested array headers can reuse the same trailing unread bytes to satisfy per-header length validation. A comparatively small payload can therefore keep many large arrays alive at once. When an attacker can submit such data repeatedly, the allocations can cause severe managed-heap pressure, excessive garbage collection, process termination, or loss of service. Exposure requires an application to deserialize untrusted data through the affected formatter path and permit repeated attacker-controlled submissions, such as a reachable ASP.NET Core SignalR hub method with an object parameter and AddMessagePackProtocol() enabled. This issue is fixed in versions 2.5.303 and 3.1.9.

Affected packages:
- Microsoft.AspNetCore.SignalR.Protocols.MessagePack (release 10.0): 10.0.0 - 10.0.12; fixed 10.0.13
- Microsoft.AspNetCore.SignalR.Protocols.MessagePack (release 8.0): 8.0.0 - 8.0.31; fixed 8.0.32
- Microsoft.AspNetCore.SignalR.Protocols.MessagePack (release 9.0): 9.0.0 - 9.0.20; fixed 9.0.21
- Microsoft.AspNetCore.SignalR.Protocols.MessagePack (release 11.0): 11.0.0-preview.5 - 11.0.0-rc.1; fixed 11.0.0-rc.2