PyO3 has a missing Sync bound on PyCFunction::new_closure closures

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

PyCFunction::newclosure (and the temporary newclosurebound 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.

Published by the source

12 Jun 2026, 20:09 UTC

Last modified by the source

12 Jun 2026, 20:09 UTC

Source last checked

26 Oct 2026, 15:42 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-chgr-c6px-7xpp

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# Patch Next advisory brief: GHSA-chgr-c6px-7xpp

Advisory: PyO3 has a missing `Sync` bound on `PyCFunction::new_closure` closures
IDs: GHSA-chgr-c6px-7xpp
Record: https://patchnext.com/advisories/ghsa-chgr-c6px-7xpp (revision 1, source checked 26 Oct 2026, 15:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pyo3 0.29.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io pyo3: affected < 0.29.0; fixed in 0.29.0

## Suggested inspection (read-only)
- crates.io pyo3: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on pyo3.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-chgr-c6px-7xpp (GitHub-reviewed advisory GHSA-chgr-c6px-7xpp lists affected ranges: crates.io pyo3 >=0 <0.29.0)

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