GHSA-9j6q-wvj5-jgjg filelock: Symlink swap during SoftFileLock's writability check leaks whether a path exists and is writable by the lock taker

• PyPI filelock: >= 3.32.6, < 4.2.1 → fixed in 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. 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 after 2 s). Reprod…