DateTime CVE-2026-44375

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