filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N

EPSS

• CVE-2026-112909: 0.22% estimated exploitation activity in the next 30 days; percentile 12.67 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

`UnixFileLock`, the backend of `FileLock` and `AsyncFileLock` on Unix, empties whatever regular file it finds at the lock path once it holds the `flock` (a kernel lock on an open file). With an explicit `mode` it also chmods that file (sets its permission bits), whether or not this acquire created it. Another local user who can rename or hard-link one of the files of the user running the lock (below: the lock user) onto the lock path gets that file truncated by the lock user's next `acquire()`. A hard link is a second name for the same file. With a mode such as `0o666` the file, still owned by the lock user, also becomes readable and writable by that other user. The other user can then read and alter everything the lock user writes to it afterwards. `O_NOFOLLOW`, the defence added for the symlink variant (CVE-2025-68146), does not see a renamed file or a hard link. The rename route needs the file's directory to be writable without the sticky bit (a directory flag that lets only a…

Published by the source

28 Oct 2026, 21:08 UTC

Last modified by the source

28 Oct 2026, 21:15 UTC

Source last checked

29 Oct 2026, 06:25 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-69hj-f68j-2879

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-69hj-f68j-2879

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# Patch Next advisory brief: CVE-2026-112909

Advisory: filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path
IDs: CVE-2026-112909, GHSA-69hj-f68j-2879
Record: https://patchnext.com/advisories/cve-2026-112909 (revision 2, source checked 29 Oct 2026, 06:25 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: filelock 4.2.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI filelock: affected < 4.2.1; fixed in 4.2.1

## Suggested inspection (read-only)
- `python -m pip show filelock`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on filelock.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-69hj-f68j-2879 (GitHub-reviewed advisory GHSA-69hj-f68j-2879 lists affected ranges: PyPI filelock >=0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-69hj-f68j-2879 (OSV record GHSA-69hj-f68j-2879 lists affected ranges: PyPI filelock >=0 <4.2.1 (fixed))

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