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Response snapshot Oct 27, 2026, 07:38 UTC

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VulnerabilityCVE-2026-112906

filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse

`SoftFileLock` and `MarkerSoftFileLock` evict an unparsable marker (the lock file at ` `, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. `SoftFileLease` expires a lease the same way.

Published Updated

At a glance

Exploited
Not known
Severity
4.7 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-2g95-wg33-c5x2 (opens a new tab)

What happened

`SoftFileLock` and `MarkerSoftFileLock` evict an unparsable marker (the lock file at ` `, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. `SoftFileLease` expires a lease the same way. The watch tells two looks apart by the marker's modification time (mtime) and inode number (the kernel's identity for a file, independent of its name) alone.

Another local user (the other user below) who can create files in the lock's directory, a sticky `/tmp` included (the sticky bit is a directory flag that lets only a file's owner remove it), parks an empty file of its own at the lock path.

It dates that file to a future tick of the kernel's coarse clock (a clock that advances in steps of 4 ms), lets the waiters of the user running the lock (the lock user below) watch it past two seconds, and unlinks it in that tick. On a filesystem that hands a freed inode number straight back (ext4, overlayfs on ext4) the lock user's next marker repeats both values.

It inherits the time spent watching the other user's file, and a second waiter removes it while its owner holds the lock.

Continue reading

Two of the lock user's processes then run the critical section together with no error. A `SoftFileLease` waiter takes a live lease that its owner wrote moments before, within milliseconds instead of after `lease_duration`. Affected versions: filelock >= 4.1.0 and /stat`, so for about 75 µs (measured) the marker exists and is empty.

A contender that polls in that gap must not take the marker for garbage, and the check that protects it is the two-second watch. `_try_break_stale_lock` reads the marker, finds no holder, and calls `break_lock_file(lock_file, mtime, ino)` only if `_marker_unchanged_for(mtime, ino)` reaches `MALFORMED_LOCK_AGE_THRESHOLD` (2.0 s). That function keeps `self.

_marker_seen = ((mtime, ino), first seen)` across polls and returns the elapsed monotonic time (time from a clock that only moves forward) while the pair repeats.

Neither half of the pair belongs to one file. A file's owner sets its mtime to any value with `utime`, a future time included, and ext4 gives the inode number of an unlinked file to the next file created in the directory.

1. While the lock is free, the other user creates an empty file at the lock path (inode `I`) and sets its mtime to `T`, a tick of the coarse clock about 2.2 s in the future. 2. Every waiting process of the lock user polls each 50 ms, reads an empty file and records `_marker_seen = ((T, I), first seen)`.

After 2 s each poll calls `break_lock_file`, whose `rename` fails with `EPERM` (operation not permitted; seen with `strace`): the directory is sticky and the file belongs to the other user. `suppress(OSError, ...)` swallows the error, the file stays, and nothing resets `_marker_seen`. 3. When the coarse clock reaches `T`, the other user unlinks its file. Inode `I` is free. 4.

Process F of the lock user polls during tick `T`. Its `O_EXCL` create succeeds, and its marker is inode `I` with mtime `T`. 5. Process S polls before F has written its record. It reads an empty file with pair `(T, I)`, `_marker_unchanged_for` returns the 2.2 s that S spent watching the other user's file, and S calls `break_lock_file(lock_file, T, I)`. 6.

`break_lock_file` compares mtime and inode before and after its rename, but with the values S just read from F's marker, so both checks pass. F's marker has S's UID, so the rename and the unlink succeed. 7. F writes its record into the unlinked inode and returns from `acquire()`. The path is empty, so the next contender to poll creates a marker and returns from `acquire()` too.

F's release finds another inode at the path (`_unlink_open_marker`) and leaves it. Neither holder learns anything.

Step 4 needs a poll inside one 4 ms tick, and step 5 a second poll inside F's 75 µs gap. Step 5 is far likelier than independent 50 ms timers would make it, because contenders that wait together bunch. With filelock's DEBUG messages timestamped, nine waiters polled within 2 to 3 ms of one another, several 0.1 ms apart or closer. In a run where the other user also recorded what replaced its file, 13 of 100 successors appeared in tick `T`, and a waiter broke 7 of those 13.

`MarkerSoftFileLock` and `SoftFileLease` hand an unparsable marker to the same code, and the lease uses the clock a second time, for expiry: `if owner.mode == "lease" and self._marker_unchanged_for(mtime, ino) >= self._lease_duration: break_lock_file(...)`. `_marker_seen` is one attribute, so the same empty file primes this branch, and step 5 has no gap to hit.

To every waiter, F's complete lease record with pair `(T, I)` has gone unrefreshed for `lease_duration`, until F's first heartbeat (the periodic refresh a lease holder runs). With the lock user's processes using `SoftFileLease(path, lease_duration=2)` and holding for 1 s, the same other user produced an overlap in 17 of 30 attempts.

The superseded holder's `on_compromise` (the callback that tells a holder it lost the lock) reported `owner-changed` 0.67 s after it had entered, at its first heartbeat; a holder that releases before that is not told at all.

## Reproduction

The scanner's reproducer for finding `ANT-2026-RKC48J5B`, as received, ran against commit b831496b0273ecd11bf02b8d080ca8b673a538c1. The `repro` script drives the files under `repro.d/`.

**`repro`**

```python #!/usr/bin/env python3 import os import shutil import subprocess import sys import time

LOCK_DIR = "/tmp/locks" LOCK_PATH = "/tmp/locks/app.lock" HOLDS_LOG = "/tmp/holds.log"

def start_victims(): # a root-owned sticky directory like /tmp, and ten processes of user victim taking turns on one lock in it shutil.rmtree(LOCK_DIR, ignore_errors=True) os.mkdir(LOCK_DIR) os.chmod(LOCK_DIR, 0o1777) victim = [sys.executable, "/tmp/repro.d/victim.py", LOCK_PATH] with open(HOLDS_LOG, "w") as holds_log: return [ subprocess.Popen(victim, stdout=holds_log, user="victim", group="victim", extra_groups=[]) for _ in

Affected package ranges reported by GitHub: filelock: >= 4.1.0, <= 4.2.0

Scores and how they were measured

CVSS base severity

4.7

Version 3.1As of

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/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 10.5%

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

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-2g95-wg33-c5x2 (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-2g95-wg33-c5x2, 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-2026-112906, scored 2026-10-26. 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 23). filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse (CVE-2026-112906). https://kevwire.com/record/CVE-2026-112906

MLA

"filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse (CVE-2026-112906)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-112906.

Chicago

KEVWire. "filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse (CVE-2026-112906)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-112906.

Evidence summary (plain text)

filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse (Oct 23, 2026).
`SoftFileLock` and `MarkerSoftFileLock` evict an unparsable marker (the lock file at ` `, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. `SoftFileLease` expires a lease the same way. The watch tells two looks apart by the marker's modification time (mtime) and inode number (the kernel's identity for a file, independent of its name) alone.

Another local user (the other user below) who can create files in the lock's directory, a sticky `/tmp` included (the sticky bit is a directory flag that lets only a file's owner remove it), parks an empty file of its own at the lock path. It dates that file to a future tick of the kernel's coarse clock (a clock that advances in steps of 4 ms), lets the waiters of the user running the lock (the lock user below) watch it past two seconds, and unlinks it in that tick. On a filesystem that hands a freed inode number straight back (ext4, overlayfs on ext4) the lock user's next marker repeats both values. It inherits the time spent watching the other user's file, and a second waiter removes it while its owner holds the lock.
CVSS 4.7 (version 3.1; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-2g95-wg33-c5x2).
EPSS 0.2% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-112906&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-2g95-wg33-c5x2: https://github.com/advisories/GHSA-2g95-wg33-c5x2
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-2026-112906&date=2026-10-26
Record ID: CVE-2026-112906.
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 27, 2026, 01:40 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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