filelock: StrictSoftFileLock creates its claim directories without the lock mode, so another local user with a write bit on them can end a live hold

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:N/I:H/A:L

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

`StrictSoftFileLock` and `AsyncStrictSoftFileLock` apply the caller's `mode` to the sentinel (the lock file at ` `) and to both claim files (the files whose presence says a process wants or holds the lock). They create the two directories that hold the claims, ` .filelock` and ` .filelock/claims`, with a bare `mkdir()`. That gives `0o777` filtered through the umask (a per-process mask that removes permission bits from new files and directories) or, under a parent with a default ACL (an access control list on a directory that sets the permissions of entries created inside it), through that ACL. A hold consists of two claim names in that directory, which has no sticky bit (a directory flag that lets only a file's owner remove it). When the environment leaves another local user (the other user below) a write bit on the directory, that user can unlink the holder's `0600` claims. Three settings do this: umask `000` in a sticky directory such as `/tmp`, umask `002` with the other user in…

Published by the source

28 Oct 2026, 21:09 UTC

Last modified by the source

28 Oct 2026, 21:15 UTC

Source last checked

29 Oct 2026, 06:10 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-6627-m3xw-93mp

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-6627-m3xw-93mp

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# Patch Next advisory brief: CVE-2026-112908

Advisory: filelock: StrictSoftFileLock creates its claim directories without the lock mode, so another local user with a write bit on them can end a live hold
IDs: CVE-2026-112908, GHSA-6627-m3xw-93mp
Record: https://patchnext.com/advisories/cve-2026-112908 (revision 2, source checked 29 Oct 2026, 06:10 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 >= 3.30.0, < 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-6627-m3xw-93mp (GitHub-reviewed advisory GHSA-6627-m3xw-93mp lists affected ranges: PyPI filelock >=3.30.0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-6627-m3xw-93mp (OSV record GHSA-6627-m3xw-93mp lists affected ranges: PyPI filelock >=3.30.0 <4.2.1 (fixed))

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