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: NuGet: MessagePack

## Summary

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.

## Impact

Applications are affected when they register `new MessagePackInputFormatter()` without explicitly passing serializer options configured for untrusted data.

An unauthenticated or otherwise untrusted HTTP client can send MessagePack request bodies that are deserialized using the trusted-data posture. For models containing hash-based collections, this can enable algorithmic complexity attacks using colliding keys. The default constructor makes the unsafe posture easy to use at the exact boundary where request bodies should be treated as untrusted.

## Affected components

- Package: `MessagePack.AspNetCoreMvcFormatter` - API: `MessagePackInputFormatter()` parameterless constructor - Scenario: ASP.NET Core MVC model binding from HTTP request bodies - Finding IDs: `MESSAGEPACKCSHARP-OPEN-009`, duplicate `MESSAGEPACKCSHARP-095`

## Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

1. Upgrade `MessagePack…