filelock: ReadWriteLock follows a symlink at the lock path into SQLite

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-2026-112936: 0.23% estimated exploitation activity in the next 30 days; percentile 14.17 (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.

Summary `ReadWriteLock` and `AsyncReadWriteLock` resolve the lock path with `pathlib.Path.resolve()` and pass it to `sqlite3.connect()` without refusing a symlink. A local user who can create names in the lock's directory, such as `/tmp` or a shared cache, can plant a symlink at the lock path and make the victim's process create, lock, or write the file it points to, or break mutual exclusion between the victim's own processes.

Details `ReadWriteLock.init` stores `pathlib.Path(lock_file).resolve()`, which follows a final symlink, and every connection opens that path through `sqlite3.connect()` with no `O_NOFOLLOW` or regular-file check. SQLite's own VFS also resolves the final symlink in user space, so `fs.protected_symlinks` and the sticky bit do not help: the kernel never sees a symlink to refuse. The other lock classes do not follow a final symlink: `FileLock` opens with `O_NOFOLLOW`, and `SoftReadWriteLock` coordinates through ` .rw/`.

Proof of concept On Linux,…

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-j8f7-rjxc-mr56

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-j8f7-rjxc-mr56

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# Patch Next advisory brief: CVE-2026-112936

Advisory: filelock: ReadWriteLock follows a symlink at the lock path into SQLite
IDs: CVE-2026-112936, GHSA-j8f7-rjxc-mr56
Record: https://patchnext.com/advisories/cve-2026-112936 (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 4.0.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI filelock: affected >= 3.20.4, < 4.0.9; fixed in 4.0.9

## 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-j8f7-rjxc-mr56 (GitHub-reviewed advisory GHSA-j8f7-rjxc-mr56 lists affected ranges: PyPI filelock >=3.20.4 <=4.0.8)
- Verified: https://osv.dev/vulnerability/GHSA-j8f7-rjxc-mr56 (OSV record GHSA-j8f7-rjxc-mr56 lists affected ranges: PyPI filelock >=3.20.4 <4.0.9 (fixed))

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