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Response snapshot Oct 29, 2026, 02:31 UTC

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VulnerabilityCVE-2026-112909

filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path

UnixFileLock, the backend of FileLock and AsyncFileLock on Unix, empties whatever regular file it finds at the lock path once it holds the flock (a kernel lock on an open file). With an explicit mode it also chmods that file (sets its permission bits), whether or not this acquire created it.

Published Updated

At a glance

Exploited
Not known
Severity
6.1 Medium (CVSS 3.1)
Chance of exploitation
0.2% in the next 30 days
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-69hj-f68j-2879 (opens a new tab)

What happened

UnixFileLock, the backend of FileLock and AsyncFileLock on Unix, empties whatever regular file it finds at the lock path once it holds the flock (a kernel lock on an open file). With an explicit mode it also chmods that file (sets its permission bits), whether or not this acquire created it.

Another local user who can rename or hard-link one of the files of the user running the lock (below: the lock user) onto the lock path gets that file truncated by the lock user's next acquire(). A hard link is a second name for the same file. With a mode such as 0o666 the file, still owned by the lock user, also becomes readable and writable by that other user.

The other user can then read and alter everything the lock user writes to it afterwards.

O_NOFOLLOW, the defence added for the symlink variant (CVE-2025-68146), does not see a renamed file or a hard link. The rename route needs the file's directory to be writable without the sticky bit (a directory flag that lets only a file's owner remove or rename it). The hard-link route into a sticky directory such as /tmp needs fs.protected_hardlinks=0, a Linux kernel setting.

Continue reading

Affected versions are every filelock release up to and including 4.2.0 (the truncation is as old as the project; the chmod since 3.10.2).

Anthropic's OSS Scanner reported this in private. Finding covered by this advisory:

- ANT-2026-KO573NWI [high, CWE-706]: UnixFileLock truncates and chmods victim files renamed onto the lock path.

Severity

CVSS 3.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N scores 6.1 (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:L (low attack complexity): one rename or link, then wait for the lock user's next acquire; no race.

I:H (high integrity impact): the lock user's own process truncates a file it did not mean to touch, including content behind a second name the other user could neither modify nor delete. With a permissive explicit mode the other user can then write to it. If that file is one the lock user later executes or trusts, that is code execution as the lock user.

C:L (partial confidentiality impact): the old content is gone before the other user can read anything, but what the lock user writes afterwards through a descriptor (the number the kernel gives a process for an open file) it still holds is readable. A:N (no availability impact). S:U (scope unchanged).

Classification from the review: security, conditional on another user controlling names in a non-sticky directory on the lock's filesystem (or on fs.protected_hardlinks=0). The scanner rated this high. The maintainers score it medium because the reachable targets depend on the directory layout and kernel settings described below.

Scope and preconditions

- Lock classes: FileLock and AsyncFileLock on Unix (UnixFileLock). SoftFileLock applies an explicit mode only to a file it created with O_CREAT | O_EXCL and does not truncate an existing file. WindowsFileLock has not truncated at any version. - The other user must put a regular file of the lock user at the lock path before the lock user's next acquire, while the lock file does not exist yet.

That is the case on first use, after a reboot, after a /tmp cleaner, or after a release with the default preserve_lock_file=False, which unlinks the file.

The scanner checked three ways at the scanned commit, with default kernel settings unless stated: - rename inside a directory without the sticky bit (a group-writable project or cache directory, a root-created 0777 directory); - rename from such a directory into a sticky one (1777) on the same filesystem. The sticky bit only protects entries that already exist, so the move succeeds while the lock path is free.

Once the lock user's lock file exists there, the same move fails with EPERM; - link(2) into a sticky directory. This needs a system that lets a user hard-link a file it cannot write, which on Linux means fs.protected_hardlinks=0. Mainstream distributions set 1, which refuses the link. The scanner did not test macOS or the BSDs.

- What the lock user loses: with the default FileLock(path) the lock truncates the file, which keeps its mode. With an explicit mode (a lock shared between users needs 0o666 or 0o660, because UnixFileLock opens the file read-write), the lock also chmods the file to that mode. _apply_explicit_mode suppresses PermissionError only for files the lock user does not own.

The lock user owns its own files, so the fchmod succeeds. - Trust boundary: the protection against another local user holds only when no other user can rename entries in the lock's directory or, for the rename-into-sticky route, in any directory on the same filesystem that holds a file of the lock user.

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. - Not affected: a symlink (a file that points at another path) at the lock path, because O_NOFOLLOW makes the open fail with ELOOP; mutual exclusion between cooperating processes.

Affected code

Scanned commit b831496b0273: [UnixFileLock._finalize_locked_fd](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_unix.py#L144-L163), whose [os.ftruncate and _apply_explicit_mode calls](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_unix.py#L150-L154) have only the st_nlink !

= 0 guard (the file still has at least one name); [_apply_explicit_mode](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_unix.py#L165-L168); [_open_lock_file](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_unix.py#L95-L121), which learns from its O_EXCL create whether this atte

Affected package ranges reported by GitHub: filelock: <= 4.2.0

Scores and how they were measured

CVSS base severity

6.1

Version 3.1As of

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

Measurement source (opens a new tab)

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

EPSS probability

0.2%

Scored Percentile 12.6%

Model and measurement source (opens a new tab)

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

Related technology

Follow Python packages (PyPI): RSS feedAll its recordsAsk your AI assistant

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-69hj-f68j-2879 (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-69hj-f68j-2879, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-112909, scored 2026-10-28. This is a model estimate, not proof of exposure.

    Published Checked

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: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path (CVE-2026-112909). https://kevwire.com/record/CVE-2026-112909

MLA

"filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path (CVE-2026-112909)." KEVWire, 28 Oct. 2026, kevwire.com/record/CVE-2026-112909.

Chicago

KEVWire. "filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path (CVE-2026-112909)." Last modified October 28, 2026. https://kevwire.com/record/CVE-2026-112909.

Evidence summary (plain text)

filelock: UnixFileLock truncates and chmods any regular file another user renames or hard-links onto the lock path (Oct 28, 2026).
UnixFileLock, the backend of FileLock and AsyncFileLock on Unix, empties whatever regular file it finds at the lock path once it holds the flock (a kernel lock on an open file). With an explicit mode it also chmods that file (sets its permission bits), whether or not this acquire created it. Another local user who can rename or hard-link one of the files of the user running the lock (below: the lock user) onto the lock path gets that file truncated by the lock user's next acquire(). A hard link is a second name for the same file.
CVSS 6.1 (version 3.1; evidence as of Oct 28, 2026; https://github.com/advisories/GHSA-69hj-f68j-2879).
EPSS 0.2% for the following 30 days, scored Oct 28, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-112909&date=2026-10-28).
Sources:
GitHub-reviewed advisory GHSA-69hj-f68j-2879: https://github.com/advisories/GHSA-69hj-f68j-2879
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-112909&date=2026-10-28
Record ID: CVE-2026-112909.
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, 02:16 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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