MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, MessagePackReader.ReadDateTime() can allocate stack memory based on an attacker-controlled MessagePack extension length. In the slow path for timestamp extension parsing, the computed tokenSize includes the extension body length from the wire and is used in a stackalloc operation before the extension length is validated as one of the valid timestamp sizes. A very small payload can claim a large timestamp extension body and cause a stack allocation large enough to trigger an uncatchable StackOverflowException, terminating the host process. This vulnerability is fixed in 2.5.301 and 3.1.7.

Affected: NuGet: MessagePack

## Summary

`MessagePackReader.ReadDateTime()` can allocate stack memory based on an attacker-controlled MessagePack extension length. In the slow path for timestamp extension parsing, the computed `tokenSize` includes the extension body length from the wire and is used in a `stackalloc` operation before the extension length is validated as one of the valid timestamp sizes.

A very small payload can claim a large timestamp extension body and cause a stack allocation large enough to trigger an uncatchable `StackOverflowException`, terminating the host process.

## Impact

Applications are affected when they deserialize untrusted payloads into types containing `DateTime` values. This path is available through the standard formatter set and does not require opting into typeless serialization, LZ4 compression, Unity-specific resolvers, or other specialized features.

`MessagePackSecurity.UntrustedData` and `MaximumObjectGraphDepth` do not mitigate this issue because the crash is caused by a single-frame stack allocation, not by object graph recursion.

An attacker can send a MessagePack timestamp extension header with an oversized body length and insufficient body bytes. The reader enters the slow path, attempts to stack-allocate a buffer sized from that declared length, and can terminate the process before a catchable serialization exception is thrown.

## Affected components

- Package: `MessagePack` - API: `MessagePackReader.ReadDateTime` - Data types: `DateTime` and formatte…