Nerdbank.MessagePack: Attacker-controlled stackalloc in DateTime decoding causes process-terminating StackOverflowException

### 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

Source published

6 May 2026 23:05:52 UTC

Source modified

14 May 2026 21:04:32 UTC

Observed here

23 Oct 2026 16:01:25 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH