Reported

June 11, 2026

Issued

June 11, 2026

Package

pyo3 (crates.io)

Type

Vulnerability

Categories

• thread-safety

Keywords

#thread-safety #unsound

References

• https://github.com/PyO3/pyo3/pull/6096

Patched

• >=0.29.0

Affected Functions

Version

pyo3::types::PyCFunction::new_closure

• >=0.15.0, <0.29.0

pyo3::types::PyCFunction::new_closure_bound

• >=0.21.0, <0.23.0

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.

Advisory available under CC0-1.0 license.