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Response snapshot Oct 29, 2026, 01:26 UTC

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VulnerabilityCVE-2026-112911

filelock: StrictSoftFileLock and SoftReadWriteLock adopt a protocol directory another user created first, losing mutual exclusion

StrictSoftFileLock, AsyncStrictSoftFileLock, SoftReadWriteLock and AsyncSoftReadWriteLock keep their whole ownership state as files in a directory derived from the lock path ( .filelock/claims, .rw/{gen,holders}).

Published Updated

At a glance

Exploited
Not known
Severity
6.3 Medium (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: filelock: 4.2.1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-9c4x-gm8v-wgc4 (opens a new tab)

What happened

StrictSoftFileLock, AsyncStrictSoftFileLock, SoftReadWriteLock and AsyncSoftReadWriteLock keep their whole ownership state as files in a directory derived from the lock path ( .filelock/claims, .rw/{gen,holders}). When that directory already exists they adopt it after checking only that it is a real directory, not who owns it.

Another local user who can create names in the lock's directory can create the tree before the first acquire on that path by the user running the lock (below: the lock user). In /tmp, /var/tmp or another sticky shared directory any user can do that; the sticky bit is a directory flag that lets only a file's owner remove or rename it. The other user then owns the tree.

They can unlink the lock user's claims while the lock user holds the lock, so a second process of the lock user, root included, enters the same critical section and neither process sees an error.

Against SoftReadWriteLock the same position also admits two writers with on_compromise silent. It redirects the lock user's own creates and renames into a directory the other user picks (the scanner replaced a repository's .git/HEAD with a filelock record written with the lock user's privileges). It also keeps the lock user's acquire or release running past any timeout. Affected versions are filelock 3.30.0 through 4.2.0 (finding ANT-2026-Q5VK5JEE).

Continue reading

Severity

CVSS 3.1 CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H scores 6.3 (Medium). AV:L (local): the other user needs an account on the same host. PR:L (low privileges): an ordinary account is enough. UI:N (no user interaction). AC:H (high attack complexity): the other user must create the protocol directory before the lock user's first acquire on that path.

That depends on the path being new, or on a reboot, a tmp cleaner or the documented .rw recovery having removed its tree; once planted, the rest needs no race. I:H (high integrity impact): two holders write whatever the lock protects, and for SoftReadWriteLock the lock user's process creates, replaces or deletes files named HEAD, epoch and 0000000000000000000N in any directory the lock user can write.

A:H (high availability impact): the owner of .rw can make the lock user's read_lock(timeout=1) spin at full CPU without end, cost 22 s per acquire, or pin 1 MiB of memory per planted file until a heartbeat thread dies of MemoryError. C:N (no confidentiality impact). S:U (scope unchanged).

Classification from the review: security, conditional on the other user winning initial ownership of the protocol directory. The scanner rated it high (CWE-283). With AC:L the same impacts would score 7.1.

Scope and preconditions

- The other user can create names in the directory that holds the lock. In a sticky 1777 directory such as /tmp any local user can. The sticky bit stops that user from renaming or removing a directory the lock user created, but does nothing for one the other user created first. - The other user gets there first: .filelock (strict) or .rw (soft read-write) must not exist when the lock user's process first acquires.

_strict.py does not remove the directory at any point, so a path the lock user has already used stays safe for as long as the tree exists. The exposure is a new lock path, or one whose tree a reboot, a tmp cleaner, a cache purge or the recovery described in docs/how-to.rst removed.

- For the strict lock the planted directory must be writable by the lock user (0777); a planted 0755 directory makes the lock user's acquire fail with PermissionError instead. Planting only the outer .filelock is enough. The lock user creates claims inside it, and the other user, as owner of the parent, renames that claims aside and creates an empty one. - The Linux fs.protected_regular, fs.protected_symlinks, fs.

protected_hardlinks and fs.protected_fifos sysctls (kernel settings, all enabled in the scanner's image) do not interfere. They cover regular files, symlinks, hard links and FIFOs, not directories. The SoftReadWriteLock symlink redirect below also passes fs.protected_symlinks=1, because the symlink (a file that points at another path) sits in the other user's own non-sticky directory.

- The affected classes are StrictSoftFileLock and AsyncStrictSoftFileLock (which inherits _acquire), and SoftReadWriteLock and AsyncSoftReadWriteLock with default options. FileLock gives way to the same position (when the other user creates the lock file first with mode 0666 and unlinks it during a hold, a second lock user process acquires), so the strict lock is not weaker than the native one here.

This advisory reports it because StrictSoftFileLock is the lock documented as fail-closed and already raises SoftFileLockProtocolError for a symlink or a regular file at the same names, while it uses a directory that belongs to someone else without any signal. The SoftReadWriteLock half has no such counterpart: its redirected creates, renames and unlinks land outside the lock's own files.

- A non-sticky directory writable by other users is outside the boundary below. There anyone who can create the directory can also rename the lock user's own tree aside, and no check inside the lock can tell that from a first use.

These protections against another local user hold only when no other user can replace the lock's directory or any of its ancestors. That holds when each ancestor belongs to the user or root and is either not writable by others or sticky. A user who can rename entries in a non-sticky directory above the lock controls every name below it, which no file-based lock can defend against.

Affected code

At the scanned commit b831496: [_ensure_protocol_directory](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_strict.py#L996-L1006) returns for any existing real directory, whatever its owner or mode. [StrictSoftFileLock._acquire](ht

Affected package ranges reported by GitHub: filelock: >= 3.30.0, <= 4.2.0

Scores and how they were measured

CVSS base severity

6.3

Version 3.1As of

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

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Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-9c4x-gm8v-wgc4 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-9c4x-gm8v-wgc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 28). filelock: StrictSoftFileLock and SoftReadWriteLock adopt a protocol directory another user created first, losing mutual exclusion (CVE-2026-112911). https://kevwire.com/record/CVE-2026-112911

MLA

"filelock: StrictSoftFileLock and SoftReadWriteLock adopt a protocol directory another user created first, losing mutual exclusion (CVE-2026-112911)." KEVWire, 28 Oct. 2026, kevwire.com/record/CVE-2026-112911.

Chicago

KEVWire. "filelock: StrictSoftFileLock and SoftReadWriteLock adopt a protocol directory another user created first, losing mutual exclusion (CVE-2026-112911)." Last modified October 28, 2026. https://kevwire.com/record/CVE-2026-112911.

Evidence summary (plain text)

filelock: StrictSoftFileLock and SoftReadWriteLock adopt a protocol directory another user created first, losing mutual exclusion (Oct 28, 2026).
StrictSoftFileLock, AsyncStrictSoftFileLock, SoftReadWriteLock and AsyncSoftReadWriteLock keep their whole ownership state as files in a directory derived from the lock path ( .filelock/claims,  .rw/{gen,holders}). When that directory already exists they adopt it after checking only that it is a real directory, not who owns it. Another local user who can create names in the lock's directory can create the tree before the first acquire on that path by the user running the lock (below: the lock user).
CVSS 6.3 (version 3.1; evidence as of Oct 28, 2026; https://github.com/advisories/GHSA-9c4x-gm8v-wgc4).
Sources:
GitHub-reviewed advisory GHSA-9c4x-gm8v-wgc4: https://github.com/advisories/GHSA-9c4x-gm8v-wgc4
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-112911.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 29, 2026, 00:55 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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