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: NuGet: MessagePack

### Impact

MessagePack-CSharp is vulnerable to denial of service through excessive memory allocation when deserializing attacker-controlled data into `System.Object`.

When `PrimitiveObjectFormatter` encounters an array, it allocates an `object[]` using the declared array length **before** enforcing the configured maximum object-graph depth. A specially crafted payload containing deeply nested arrays can therefore keep many large arrays live simultaneously before deserialization is rejected for exceeding the depth limit.

Because the array-header validation uses the total unread byte count as evidence that the declared elements may be present, nested array headers can reuse the same trailing bytes for this validation. This permits a relatively small payload to cause disproportionately large managed-heap allocations. The reported example uses an approximately 31.5 KiB payload to allocate roughly 114.7 MiB of reference storage on x64—an amplification factor of approximately 3,800—before the depth exception is raised. Repeated requests may cause severe memory pressure, excessive garbage collection, process termination, or loss of service availability.

Applications are affected when they:

- Deserialize untrusted MessagePack data as `object`, or otherwise use a resolver path that selects `PrimitiveObjectFormatter`; and - Permit an attacker to submit such data repeatedly.

ASP.NET Core SignalR is one known way to expose the vulnerability remotely: an application that enables `A…