filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory

Exploitation

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

CVSS

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

EPSS

• CVE-2026-112914: 0.16% estimated exploitation activity in the next 30 days; percentile 5.34 (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.

When a soft-lock stale break finds that the marker (the lock file at <path) it renamed aside was a live one recreated by a successor, the restore branch of breaklockfile() checks the break name with lstat (a check that reads a name without following a symbolic link) and then calls os.link(breakpath, lockfile) by name. On CPython 3.12 and later on Linux (and on macOS) that call follows a symbolic link (a file that redirects to another path). A local user (the other user below) who can write a shared lock directory without the sticky bit (a directory flag that lets only a file's owner rename or remove it) replaces the break name with a symbolic link between those two calls. The SoftFileLock, MarkerSoftFileLock, SoftFileLease or lifetime break of the user running the lock (the lock user below) then creates a hard link (a second name for the same file), in that directory, to any of the lock user's own files on the same filesystem. That includes files inside directories the other user…

Published by the source

23 Oct 2026, 22:00 UTC

Last modified by the source

23 Oct 2026, 22:15 UTC

Source last checked

26 Oct 2026, 13:55 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-crv4-6mhj-j5jh

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-crv4-6mhj-j5jh

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-crv4-6mhj-j5jh

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# Patch Next advisory brief: CVE-2026-112914

Advisory: filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory
IDs: CVE-2026-112914, GHSA-crv4-6mhj-j5jh
Record: https://patchnext.com/advisories/cve-2026-112914 (revision 3, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
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 >= 4.1.0, <= 4.2.0; 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-crv4-6mhj-j5jh (GitHub-reviewed advisory GHSA-crv4-6mhj-j5jh lists affected ranges: PyPI filelock >=4.1.0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-crv4-6mhj-j5jh (OSV record GHSA-crv4-6mhj-j5jh lists affected ranges: PyPI filelock >=4.1.0 <4.2.1 (fixed))

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