AIKIDO-2026-858049
MessagePack is vulnerable to Deserialization of Untrusted Data
59
Medium Risk
This Affects:
MessagePackTL;DR
MessagePack's Typeless (dynamic) serialization mode resolves an embedded type name from the payload and instantiates the corresponding .NET type, gated only by the default DisallowedTypes denylist in MessagePackSerializerOptions. Before this fix, that denylist omits several classes commonly used as .NET deserialization gadgets, including ObjectDataProvider, ClaimsIdentity, and SessionSecurityToken, so untrusted MessagePack payloads processed with Typeless deserialization can instantiate those types and reach known gadget behavior. The fix adds these types to the default DisallowedTypes set so TypelessFormatter rejects them during deserialization.
Who does this affect?
You are affected if you are using a version that falls within the vulnerable range and you deserialize untrusted MessagePack data using Typeless (dynamic) type resolution.
Background info
MessagePack is vulnerable to Deserialization of Untrusted Data in versions 1.4.0 - 2.5.198 and 3.0.3 - 3.1.4.
How to fix this
Upgrade the MessagePack library to the patch version.
Are You Affected?
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