Messagepack Messagepack — CVE-2026-48509 (Critical)

Summary

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 vulnerab

Why it matters

This CVE carries a CRITICAL severity rating (CVSS 9.1) in Messagepack Messagepack. Patch or mitigate promptly. EPSS percentile: 13%.

Operator check

Review CVE-2026-48509 in your asset inventory. Apply patches per vendor guidance and verify Messagepack is not exposed. CVSS score: 9.1. EPSS probability: 0.2%; percentile: 13%.

Sources