# CVE-2026-112935: filelock SoftFileLock file lock bypass

Source: https://junglewise.ai/threats/cve-2026-112935-filelock-is-a-platform-independent-file-lock-for-python-from-3  
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-2026-112935 · Severity: medium · CVSS 4.7 · Published 2026-10-20

filelock is a Python library that provides platform-independent file locking for coordinating access to shared resources. A vulnerability in the SoftFileLock mechanism allows an attacker to manipulate stale lock markers in a way that causes the locking mechanism to incorrectly admit multiple concurrent holders to the same resource, potentially 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-2026-112935 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-112935 being exploited in the wild.

**How severe is CVE-2026-112935?** Medium severity, with a CVSS score of 4.7, published 20 October 2026.

**Which products and versions are affected?** filelock (PyPI) >= 3.22.0, <= 4.2.0, ECOSYSTEM:introduced=3.22.0,fixed=4.2.1; Tox-Dev Filelock 3.22.0 to 4.2.0.

**Is there a fix for CVE-2026-112935?** 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-2026-112935 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-112935?** 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

The vulnerability exists in filelock versions 3.22.0 through 4.2.1 in the SoftFileLock class. An attacker can break a stale marker after freeing their own marker, causing a live successor created on a reused inode to be incorrectly removed if its modification time is not newer than a saved observation, allowing two lock holders to be admitted simultaneously. The attack requires local access to manipulate file system markers and timing conditions.

## Affected products

- tox-dev filelock 3.22.0 to 4.2.0

## Identifiers

- CVE-2026-112935
- GHSA-fq56-3cx3-fjxp

## Timeline

- 2026-10-20: disclosed
- 2026-10-17: patched: Fix committed and merged

## References

- [https://github.com/tox-dev/filelock/commit/740ff666b62a88496c8d7394e4e6230605e4d62b](https://github.com/tox-dev/filelock/commit/740ff666b62a88496c8d7394e4e6230605e4d62b)
- [https://github.com/tox-dev/filelock/pull/843](https://github.com/tox-dev/filelock/pull/843)

## 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)