GHSA-2cwq-pwfr-wcw3

### Summary

Nerdbank.MessagePack contains an uncontrolled stack allocation vulnerability in DateTime decoding. A malicious MessagePack payload can declare an oversized timestamp extension length, causing the reader to allocate an attacker-controlled number of bytes on the stack. This can trigger a `StackOverflowException`, which is not catchable by user code and terminates the process.

### Impact

Applications are impacted if they deserialize MessagePack data from untrusted or attacker-controlled sources using Nerdbank.MessagePack and the target type contains a `DateTime` value.

A small malicious payload can cause process termination, resulting in a denial of service. This may affect services, APIs, workers, message consumers, or other long-running processes that deserialize untrusted MessagePack input.

The issue occurs because DateTime timestamp extension decoding derives `tokenSize` from the attacker-controlled extension length before validating that the timestamp length is one of the legal MessagePack timestamp sizes: 4, 8, or 12 bytes. When the buffer is incomplete, that unvalidated size is propagated to the streaming reader slow path, where it is used in a `stackalloc`.

### Patches

The 1.1.62 version contains the fix for this security vulnerability.

### Workarounds

If upgrading is not yet possible, avoid deserializing untrusted MessagePack payloads into type graphs that may contain `DateTime` fields or properties.

Input byte-size limits alone may not fully mitigate this issue, because the malicious payload can be small while declaring a very large extension length. Possible mitigations include:

- Pre-validating MessagePack extension headers before deserialization and rejecting timestamp extensions whose length is not 4, 8, or 12 bytes. - Rejecting or filtering extension type `-1` timestamp values from untrusted input unless they are known to be valid. - Running deserialization of untrusted payloads in an isolated process that can be safely restarted after termination. - Restricting MessagePack deserialization to trusted producers until a patched version is available.

### Resources

- CWE-789: Uncontrolled Memory Allocation: https://cwe.mitre.org/data/definitions/789.html - MessagePack timestamp extension specification: https://github.com/msgpack/msgpack/blob/master/spec.md#timestamp-extension-type