filelock: ReadWriteLock on the hard-link route deletes any directory another user renames onto its .filelock-* link directory name

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:N/I:H/A:L

EPSS

• CVE-2026-112937: 0.16% estimated exploitation activity in the next 30 days; percentile 5.42 (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.

ReadWriteLock and AsyncReadWriteLock (the SQLite-backed reader/writer lock) remove their private .filelock- link directories with shutil.rmtree (a recursive delete) on a remembered path. They do not check that the path still names the directory they created. This happens on the hard-link route (a hard link is a second name for the same file). That route is every connection on macOS since 4.1.1 and any POSIX process without a usable /proc/self/fd (a Linux path that names the process's own open files), such as a chroot or container that does not mount /proc. Another local user who can rename entries in the directory that holds the lock database can rename one of the directories of the user running the lock (below: the lock user) onto the link directory's name, or onto a forged dead-holder name. Such a lock directory is a 0777 or group-writable cache, build or project directory without the sticky bit (a directory flag that lets only a file's owner remove or rename it). The lock user's…

Published by the source

23 Oct 2026, 22:28 UTC

Last modified by the source

23 Oct 2026, 22:45 UTC

Source last checked

26 Oct 2026, 13:56 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jqc4-5cvw-7w85

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-jqc4-5cvw-7w85

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-jqc4-5cvw-7w85

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# Patch Next advisory brief: CVE-2026-112937

Advisory: filelock: ReadWriteLock on the hard-link route deletes any directory another user renames onto its .filelock-* link directory name
IDs: CVE-2026-112937, GHSA-jqc4-5cvw-7w85
Record: https://patchnext.com/advisories/cve-2026-112937 (revision 3, source checked 26 Oct 2026, 13:56 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 >= 4.1.0, <= 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-jqc4-5cvw-7w85 (GitHub-reviewed advisory GHSA-jqc4-5cvw-7w85 lists affected ranges: PyPI filelock >=4.1.0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-jqc4-5cvw-7w85 (OSV record GHSA-jqc4-5cvw-7w85 lists affected ranges: PyPI filelock >=4.1.0 <4.2.1 (fixed))

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