RUSTSEC-2026-0177: Missing `Sync` bound on `PyCFunction::new_closure` closures

What to do

1. See which version you have: cargo tree -i pyo3 (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p pyo3. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example pyo3 = "0.29.0") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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.

References

• Package: crates.io/crates/pyo3
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0177.html
• Web: github.com/PyO3/pyo3/pull/6096

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for pyo3

• Vulnerability MediumRUSTSEC-2020-0074 · 2020-11-28Reference counting error in `From<Py<T>>`
• VulnerabilityRUSTSEC-2024-0378 · 2024-10-12Risk of use-after-free in `borrowed` reads from Python weak references
• VulnerabilityRUSTSEC-2024-0409 · 2024-12-04Build corruption when using `PYO3_CONFIG_FILE` environment variable
• VulnerabilityRUSTSEC-2025-0020 · 2025-04-01Risk of buffer overflow in `PyString::from_object`
• VulnerabilityRUSTSEC-2026-0013 · 2026-02-18Type confusion when accessing data from sublasses of subclasses of native types with `abi3` feature targeting Python 3.12 and up
• VulnerabilityRUSTSEC-2026-0176 · 2026-06-11Out-of-bounds read in `nth` / `nth_back` for `PyList` and `PyTuple` iterators

Full security history of pyo3