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Medium, CVSS 5.8amended

filelock: StrictSoftFileLock follows a symbolic link swapped in at .filelock and creates claim files in a directory another local user chooses

CVE-2026-112938GHSA-m462-7rxw-wvpg

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (filelock 4.2.1), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI filelock

    Affected
    >= 3.30.0, < 4.2.1
    Fixed in
    4.2.1

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
5.8 · CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

`StrictSoftFileLock` and `AsyncStrictSoftFileLock` check ` .filelock` (the coordination directory that holds the lock's state) with one `lstat` call on its name. `lstat` reports what a name is without following a symbolic link (a file that redirects to another path). After that check, the code reaches ` .filelock/claims` by pathname for every later operation (`mkdir`, scan, publish, link, unlink). `_open_directory` passes `O_NOFOLLOW` (a flag that makes an open fail when the final name is a symbolic link), but the flag covers only the final `claims` component. The kernel follows a symbolic link in any earlier component. Another local user (the other user below) who can rename ` .filelock` exchanges it for a symbolic link after the check. In a shared directory without the sticky bit (a directory flag that lets only a file's owner rename or remove it), any user with write access can do this. Where the shared directory is sticky, such as `/tmp`, only the user who created ` .filelock`…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-m462-7rxw-wvpg

  2. Revision 1 · created ·

    Published from ghsa record GHSA-m462-7rxw-wvpg

Agent brief

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# Patch Next advisory brief: CVE-2026-112938

Advisory: filelock: StrictSoftFileLock follows a symbolic link swapped in at .filelock and creates claim files in a directory another local user chooses
IDs: CVE-2026-112938, GHSA-m462-7rxw-wvpg
Record: https://patchnext.com/advisories/cve-2026-112938 (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 5.8)
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-m462-7rxw-wvpg (GitHub-reviewed advisory GHSA-m462-7rxw-wvpg lists affected ranges: PyPI filelock >=3.30.0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-m462-7rxw-wvpg (OSV record GHSA-m462-7rxw-wvpg lists affected ranges: PyPI filelock >=3.30.0 <4.2.1 (fixed))

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