msgpack-python C Extension SIGSEGV — PoC

Vulnerability

msgpack-python's C extension (_cmsgpack) has no recursion depth limit when calling user-provided ext_hook callbacks. When an application (like orbax-checkpoint) uses a recursive ext_hook, a crafted msgpack payload with ~300 nesting levels causes a native stack overflow → SIGSEGV → process crash.

Impact

orbax-checkpoint (Google/JAX checkpoint library) uses recursive ext_hook
Any JAX/Flax application loading orbax checkpoints is vulnerable
A 1.1 KB payload crashes the process with SIGSEGV (no error handling possible)
DoS from a crafted model checkpoint file

Affected

msgpack (PyPI) ≤ 1.1.2 — C extension has no depth limit
orbax-checkpoint (PyPI) ≤ 0.11.34 — recursive ext_hook in msgpack_utils.py line 113-116

Files

poc.py — Full PoC with 3 tests (SIGSEGV, orbax crash, Python fallback safe)
crash_payload.msgpack — 1117-byte crafted payload

Reproduce

pip install msgpack orbax-checkpoint flax
python poc.py

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