🛡️ RUSTSEC-2026-0177 — pyo3

Missing Sync bound on PyCFunction::new_closure closures

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.