# CVE-2026-112906: filelock SoftFileLock stale marker reuse race condition

Source: https://junglewise.ai/threats/cve-2026-112906-filelock-is-a-platform-independent-file-lock-for-python-from-4-1  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-20. Updated: 2026-10-20  
License: CC BY 4.0 with a link back

CVE-2026-112906 · Severity: medium · CVSS 4.7 · Published 2026-10-20

filelock is a Python library that manages file-based locks to coordinate access to shared resources across multiple processes. A flaw in SoftFileLock versions 4.1.0 through 4.2.0 allows an attacker to trick the locking mechanism into incorrectly removing a legitimate holder's marker, potentially causing one process to lose its lock and another to proceed without proper synchronization, leading to data corruption or loss.

Technologies: [filelock (PyPI)](https://junglewise.ai/threats/technologies/filelock), [Tox-Dev Filelock](https://junglewise.ai/threats/technologies/tox-dev-filelock). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi), [Tox-Dev](https://junglewise.ai/threats/vendors/tox-dev).

## Quick answers

**Is CVE-2026-112906 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-112906 being exploited in the wild.

**How severe is CVE-2026-112906?** Medium severity, with a CVSS score of 4.7, published 20 October 2026.

**Which products and versions are affected?** filelock (PyPI) >= 4.1.0, <= 4.2.0, ECOSYSTEM:introduced=4.1.0,fixed=4.2.1; Tox-Dev Filelock 4.1.0 to 4.2.0.

**Is there a fix for CVE-2026-112906?** Yes. filelock (PyPI): upgrade to 4.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-112906 attacked?** It is attacked with local access.

**Am I affected by CVE-2026-112906?** Check whether you run filelock (PyPI) or Tox-Dev Filelock at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- filelock (PyPI): upgrade to 4.2.1 or later

  ```sh
  pip install "filelock>=4.2.1"
  ```


## Technical details

SoftFileLock maintains lock state via marker files on the filesystem. The vulnerability is a race condition where an attacker can pre-date and remove their own marker, then exploit inode reuse when multiple processes are waiting and filesystem modification times align, causing the lock mechanism to incorrectly classify a fresh, live marker as stale and remove it. This breaks the lock's mutual exclusion guarantee, allowing concurrent access to protected resources. The issue is fixed in version 4.2.1 by using atomic marker exchange operations on platforms that support them.

## Affected products

- tox-dev filelock 4.1.0 to 4.2.0

## Identifiers

- CVE-2026-112906
- GHSA-2g95-wg33-c5x2

## Timeline

- 2026-10-20: disclosed
- 2026-10-17: patched: fix commit date

## References

- [https://github.com/tox-dev/filelock/commit/740ff666b62a88496c8d7394e4e6230605e4d62b](https://github.com/tox-dev/filelock/commit/740ff666b62a88496c8d7394e4e6230605e4d62b)
- [https://github.com/tox-dev/filelock/releases/tag/4.2.1](https://github.com/tox-dev/filelock/releases/tag/4.2.1)

## Related threats

- [filelock StrictSoftFileLock symlink race privilege escalation](https://junglewise.ai/threats/cve-2026-112938-filelock-strictsoftfilelock-follows-a-symbolic-link-swapped-in)
- [filelock: ReadWriteLock on Windows acts on SQLite side files another local user places beside its database](https://junglewise.ai/threats/cve-2026-112913-filelock-readwritelock-on-windows-acts-on-sqlite-side-files)
- [filelock WindowsFileLock mode parameter permission enforcement failure](https://junglewise.ai/threats/cve-2026-112910-filelock-windowsfilelock-ignores-mode-a-0o600-lock-file-and-the)
- [filelock StrictSoftFileLock directory permission bypass](https://junglewise.ai/threats/cve-2026-112908-filelock-strictsoftfilelock-creates-its-claim-directories)
- [filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break](https://junglewise.ai/threats/cve-2026-112907-filelock-on-windows-an-oplock-on-a-soft-lock-s-marker-holds)