MessagePack for C# is a MessagePack serializer for C#

Overview

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.

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Package advisory (CVE-2026-92707)

Affected packages:

• MessagePack < 2.5.303
• MessagePack >= 3.0.0, < 3.1.9

Patched in:

• MessagePack 2.5.303
• MessagePack 3.1.9

Source: https://github.com/advisories/GHSA-qhrr-8q5h-9q3h