# 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.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Not stated
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Jun 2026
- **Also known as:** RUSTSEC-2026-0177

## 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 | Ecosystem | 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.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-chgr-c6px-7xpp and fix it if it is affected.

What it is: GHSA-chgr-c6px-7xpp is a vulnerability in pyo3, published on 11 Jun 2026. 
Exploitation: There is no known exploitation of GHSA-chgr-c6px-7xpp in the wild and no public exploit code.

Affected packages and fixed versions:
- pyo3 (crates.io): affected all versions; fixed in 0.29.0
- pyo3 (crates.io): affected all versions; fixed in 0.29.0
- pyo3 (crates.io): affected from 0.0.0-0; fixed in 0.29.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-chgr-c6px-7xpp/ (Markdown: https://insights.sbomify.com/GHSA-chgr-c6px-7xpp/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-chgr-c6px-7xpp

---
From sbomify Insights: https://insights.sbomify.com/GHSA-chgr-c6px-7xpp/
Summaries are generated and may contain mistakes; the official description above is the record.