CVE-2026-48511: Messagepack

High severity, CVSS 7.5. 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, ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object>.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies. For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals. 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.