Tox-dev Filelock: Link Following (CVE-2026-112912)

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.

Severity

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.

Scope and preconditions

• The affected classes are 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.
• The other user must be able to create entries in the lock's directory. With Linux defaults (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).
• The swap is practical only with a second CPU for the other user.
• The other user needs to see how the lock user's acquire ended, for example the exception in a shared log, an exit status, or when the job stopped (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.
• The O_NOFOLLOW open that follows the check is not affected. The lock does not read, create or modify anything through the symlink.

Affected code

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.

Introduced

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.

Mechanism

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 afte