CVE-2026-48509: Messagepack

Critical severity, CVSS 9.1. EPSS: 0.4% chance of exploitation in the next 30 days.

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, the parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with MessagePackSecurity.TrustedData. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary. This insecure default can expose applications to denial-of-service attacks that MessagePackSecurity.UntrustedData is intended to mitigate, such as hash-collision attacks against dictionary-like model properties. This vulnerability is fixed in 2.5.301 and 3.1.7.

Affected products

• Messagepack Messagepack: before 2.5.301 (fixed in 2.5.301); from 3.0.3, before 3.1.7 (fixed in 3.1.7)

Published 2026-06-22. Last modified 2026-06-25.