GHSA-chgr-c6px-7xpp Missing `Sync` bound on `PyCFunction::new_closure` closures
Missing Sync bound in PyO3 allows concurrent closure execution, potentially causing data races and memory corruption in multi-threaded Python environments.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Not stated
- Victim action needed
- Not needed
- Fix
- Available
What is GHSA-chgr-c6px-7xpp?
GHSA-chgr-c6px-7xpp is a vulnerability in pyo3, published on 11 Jun 2026.
Official description
PyCFunction::new_closure (and the temporary new_closure_bound complement in
the 0.21–0.22 series) required the supplied closure to be Send + 'static but
not Sync. The resulting PyCFunction is a Python callable that can be
invoked from any Python thread, which means the closure may be called
concurrently from multiple threads, and needs a Sync bound to prevent
possible data races.
The problem exists under all Python versions but is particularly vulnerable under
the newer free-threaded Python variant, which do not have serial execution
imposed by the Global Interpreter Lock. Under releases protected by the GIL,
the ability to "detach" from the Python interpreter temporarily inside the closure
(e.g. by Python::detach) makes it possible for interleaved and/or concurrent
execution of various portions of the closure.
PyO3 0.29.0 added a Sync bound to close this thread-safety bug.
Which versions are affected by GHSA-chgr-c6px-7xpp?
3 open source packages ship an affected version, including pyo3.
| Package | Affected | Fixed in |
|---|---|---|
pyo3 crates.io | All versions | 0.29.0 |
pyo3 crates.io | All versions | 0.29.0 |
pyo3 crates.io | From 0.0.0-0 | 0.29.0 |
How do I fix GHSA-chgr-c6px-7xpp?
Upgrade pyo3 to 0.29.0 or later.
Is GHSA-chgr-c6px-7xpp being exploited?
There is no known exploitation of GHSA-chgr-c6px-7xpp in the wild and no public exploit code.
Sources
Also known as RUSTSEC-2026-0177.