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OpenAdvisories
Medium · 6.3 CVE-2026-48509 EUVD-2026-38387GHSA-2f33-pr97-265q

MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies

Published Jun 22, 2026 · Updated Jun 25, 2026 · Assigned by GitHub_M

Description

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

VendorProductAffectedFixed
MessagePack-CSharp MessagePack-CSharp >= 3.0, < 3.1.7, < 2.5.301 —

Fixed in Linux distributions

Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.

Azure Linux

ReleasePackageFixed inAdvisory
Azure Linux 3 python-msgpack No fix yet —

Affected packages

EcosystemPackageAffectedFixedRecord
NuGet MessagePack < 2.5.301 2.5.301 GHSA-2f33-pr97-265q
NuGet MessagePack >= 3.0, < 3.1.7 3.1.7 GHSA-2f33-pr97-265q

References