# CVE-2026-112939: filelock SoftFileLock PID spoofing privilege escalation

Source: https://junglewise.ai/threats/cve-2026-112939-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-2026-112939 · Severity: medium · CVSS 4.7 · Published 2026-10-20

filelock is a Python library providing cross-platform file locking for coordinating access to shared resources. A vulnerability in SoftFileLock allows an attacker with local access to forge process identifiers in lock markers, trick the library into believing a lock is free when it is not, and acquire locks held by other processes. This could lead to unauthorized access to resources, data corruption, or privilege escalation if combined with other vulnerabilities.

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-2026-112939 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-112939 being exploited in the wild.

**How severe is CVE-2026-112939?** 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-2026-112939?** Yes. filelock (PyPI): upgrade to 4.2.1 or later. Until then: Ensure the directory containing lock markers has the sticky bit set to prevent unauthorized file replacement by other users. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-112939 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-112939?** 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"
  ```


Ensure the directory containing lock markers has the sticky bit set to prevent unauthorized file replacement by other users.

## Technical details

SoftFileLock determines lock availability by reading marker files and checking if the recorded process owner is alive. A flaw in marker validation allows an attacker to forge the NSpid line in /proc/&lt;pid&gt;/status by injecting a carriage return in the process name, causing the library to misread process identity and incorrectly consider a live lock holder as dead. Combined with the ability to modify marker content, an authorized local user can remove live markers and acquire locks they should not hold.

## Affected products

- tox-dev filelock 3.22.0 to 4.2.0

## Identifiers

- CVE-2026-112939
- GHSA-rfpc-qcw5-3pc4

## Timeline

- 2026-10-20: disclosed
- 2026-10-17: patched

## References

- [https://github.com/tox-dev/filelock/commit/eeceaa846cbe4df72718d94c8371ca1ab8f5b3cc](https://github.com/tox-dev/filelock/commit/eeceaa846cbe4df72718d94c8371ca1ab8f5b3cc)
- [https://github.com/tox-dev/filelock/pull/836](https://github.com/tox-dev/filelock/pull/836)

## 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)