filelock: ReadWriteLock follows a symlink at the lock path into SQLite

Summary

ReadWriteLock and AsyncReadWriteLock resolve the lock path with pathlib.Path.resolve() and pass it to sqlite3.connect() without refusing a symlink. A local user who can create names in the lock's directory, such as /tmp or a shared cache, can plant a symlink at the lock path and make the victim's process create, lock, or write the file it points to, or break mutual exclusion between the victim's own processes.

Details

ReadWriteLock.__init__ stores pathlib.Path(lock_file).resolve(), which follows a final symlink, and every connection opens that path through sqlite3.connect() with no O_NOFOLLOW or regular-file check. SQLite's own VFS also resolves the final symlink in user space, so fs.protected_symlinks and the sticky bit do not help: the kernel never sees a symlink to refuse. The other lock classes do not follow a final symlink: FileLock opens with O_NOFOLLOW, and SoftReadWriteLock coordinates through <path>.rw/.

Proof of concept

On Linux, with the victim and attacker as different users and /shared mode 1777 with fs.protected_symlinks=1:

1. The attacker runs ln -s /home/victim/private/pwned.flag /shared/app.rwlock.db.
2. The victim constructs ReadWriteLock("/shared/app.rwlock.db"). A plain os.open(..., O_NOFOLLOW) on the same path fails, but the lock creates /home/victim/private/pwned.flag, owned by the victim, inside a directory the attacker cannot write.
3. With /shared/cache.rwlock.db -> /shared/a.db, one victim process holds the write lock. The attacker repoints the link to /shared/b.db, and a second victim process acquires the write lock while the first still holds it.

The same behavior reproduces on macOS, on Windows 11, and through AsyncReadWriteLock.

Impact

A local attacker with write access to the lock directory can make the victim create empty files it owns at paths the attacker cannot write, make the victim hold a SQLite lock on another application's database so that application gets database is locked, and give two victim processes the write lock at once, letting them corrupt the resource the lock protects.

Workarounds

Keep ReadWriteLock paths in directories that only trusted users can write.