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Medium, CVSS 4.7amended

filelock: SoftFileLock removes a live holder's marker when another user forges the PID in a writable marker

CVE-2026-112939GHSA-rfpc-qcw5-3pc4

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
    >= 3.22.0, < 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
4.7 · CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N
EPSS
  • CVE-2026-112939: 0.14% estimated exploitation activity in the next 30 days; percentile 3.34 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

`SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease`, their async variants and a `FileLock` that has fallen back to a soft lock on `ENOSYS` decide from the record inside the marker file whether its owner is gone. The marker is the file whose presence says a process holds the lock. They do not check who could have written that record. A local user who can write the marker but cannot remove it rewrites the holder's record to name a dead PID (process id). That position arises with an explicit `mode` with group or other write bits, or the default mode under a lax umask (the process setting that strips permission bits from new files), in a sticky or otherwise non-removable directory. The sticky bit is a directory flag that lets only a file's owner remove or rename it. The next contender running as the holder's account, or root, judges the lock stale, removes the live marker on that user's behalf and acquires it. Two processes then run the critical section together and neither gets an…

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

    Amended from osv record GHSA-rfpc-qcw5-3pc4

  2. Revision 1 · created ·

    Published from ghsa record GHSA-rfpc-qcw5-3pc4

Agent brief

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# Patch Next advisory brief: CVE-2026-112939

Advisory: filelock: SoftFileLock removes a live holder's marker when another user forges the PID in a writable marker
IDs: CVE-2026-112939, GHSA-rfpc-qcw5-3pc4
Record: https://patchnext.com/advisories/cve-2026-112939 (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 4.7)
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 >= 3.22.0, < 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-rfpc-qcw5-3pc4 (GitHub-reviewed advisory GHSA-rfpc-qcw5-3pc4 lists affected ranges: PyPI filelock >=3.22.0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-rfpc-qcw5-3pc4 (OSV record GHSA-rfpc-qcw5-3pc4 lists affected ranges: PyPI filelock >=3.22.0 <4.2.1 (fixed))

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