RUSTSEC-2026-0177

    Dashboard / Vulnerabilities / RUSTSEC-2026-0177

    RUSTSEC-2026-0177

    Published: 11 Jun 2026Last Modified: 13 Jun 2026

    Summary: Missing `Sync` bound on `PyCFunction::new_closure` closures

    Details: `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: https://crates.io/crates/pyo3, https://rustsec.org/advisories/RUSTSEC-2026-0177.html, https://github.com/PyO3/pyo3/pull/6096

    Affected packages

    Package

    Name: pyo3

    Purl: pkg:cargo/pyo3

    Affected ranges

    Type: SEMVER

    Events:

    Introduced- 0.0.0-0
    Fixed -0.29.0

    Affected versions

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    RUSTSEC-2026-0177 | CVE-DB