CVE-2026-92707MessagePack: Nested arrays can blow past buffer length limits

Official description

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 & fixed versions

Affected products (as reported)

References

• https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-qhrr-8q5h-9q3h x_refsource_CONFIRM
• https://github.com/MessagePack-CSharp/MessagePack-CSharp/commit/7fd78bbf3476e03beae8b0f7a521125525a8a64a x_refsource_MISC
• https://github.com/MessagePack-CSharp/MessagePack-CSharp/commit/95b63fca4f7097d3cac12d1a95286af4b4bbf02c x_refsource_MISC
• https://github.com/MessagePack-CSharp/MessagePack-CSharp/releases/tag/v2.5.303 x_refsource_MISC
• https://github.com/MessagePack-CSharp/MessagePack-CSharp/releases/tag/v3.1.9 x_refsource_MISC