Tox-dev Filelock: Unverified Ownership (CVE-2026-112911)

StrictSoftFileLock, AsyncStrictSoftFileLock, SoftReadWriteLock and AsyncSoftReadWriteLock keep their whole ownership state as files in a directory derived from the lock path (<path>.filelock/claims, <path>.rw/{gen,holders}). When that directory already exists they adopt it after checking only that it is a real directory, not who owns it. Another local user who can create names in the lock's directory can create the tree before the first acquire on that path by the user running the lock (below: the lock user). In /tmp, /var/tmp or another sticky shared directory any user can do that; the sticky bit is a directory flag that lets only a file's owner remove or rename it. The other user then owns the tree. They can unlink the lock user's claims while the lock user holds the lock, so a second process of the lock user, root included, enters the same critical section and neither process sees an error.

Against SoftReadWriteLock the same position also admits two writers with on_compromise silent. It redirects the lock user's own creates and renames into a directory the other user picks (the scanner replaced a repository's .git/HEAD with a filelock record written with the lock user's privileges). It also keeps the lock user's acquire or release running past any timeout. Affected versions are filelock 3.30.0 through 4.2.0 (finding ANT-2026-Q5VK5JEE).

Severity

CVSS 3.1 CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H scores 6.3 (Medium). 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 other user must create the protocol directory before the lock user's first acquire on that path. That depends on the path being new, or on a reboot, a tmp cleaner or the documented <path>.rw recovery having removed its tree; once planted, the rest needs no race. I:H (high integrity impact): two holders write whatever the lock protects, and for SoftReadWriteLock the lock user's process creates, replaces or deletes files named HEAD, epoch and 0000000000000000000N in any directory the lock user can write. A:H (high availability impact): the owner of <path>.rw can make the lock user's read_lock(timeout=1) spin at full CPU without end, cost 22 s per acquire, or pin 1 MiB of memory per planted file until a heartbeat thread dies of MemoryError. C:N (no confidentiality impact). S:U (scope unchanged).

Classification from the review: security, conditional on the other user winning initial ownership of the protocol directory. The scanner rated it high (CWE-283). With AC:L the same impacts would score 7.1.

Scope and preconditions

• The other user can create names in the directory that holds the lock. In a sticky 1777 directory such as /tmp any local user can. The sticky bit stops that user from renaming or removing a directory the lock user created, but does nothing for one the other user created first.
• The other user gets there first: <path>.filelock (strict) or <path>.rw (soft read-write) must not exist when the lock user's process first acquires. _strict.py does not remove the directory at any point, so a path the lock user has already used stays safe for as long as the tree exists. The exposure is a new lock path, or one whose tree a reboot, a tmp cleaner, a cache purge or the recovery described in docs/how-to.rst removed.
• For the strict lock the planted directory must be writable by the lock user (0777); a planted 0755 directory makes the lock user's acquire fail with PermissionError instead. Planting only the outer <path>.filelock is enough. The lock user creates claims inside it, and the other user, as owner of the parent, renames that claims aside and creates an empty one.
• The Linux fs.protected_regular, fs.protected_symlinks, fs.protected_hardlinks and fs.protected_fifos sysctls (kernel settings, all enabled in the scanner's image) do not interfere. They cover regular files, symlinks, hard links and FIFOs, not directories. The SoftReadWriteLock symlink redirect below also passes fs.protected_symlinks=1, because the symlink (a file that points at another path) sits in the other user's own non-sticky directory.
• The affected classes are StrictSoftFileLock and AsyncStrictSoftFileLock (which inherits _acquire), and SoftReadWriteLock and AsyncSoftReadWriteLock with default options. FileLock gives way to the same position (when the other user creates the lock file first with mode 0666 and unlinks it during a hold, a second lock user process acquires), so the strict lock is not weaker than the native one here. This advisory reports it because StrictSoftFileLock is the lock documented as fail-closed and already raises SoftFileLockProtocolError for a symlink or a regular file at the same names, while it uses a directory that belongs to someone else without any signal. The SoftReadWriteLock half has no such counterpart: its redirected creates, renames and unlinks land outside the lock's own files.
• A non-sticky directory writable by other users is outside the boundary below. There anyone who can create the directory can also rename the lock user's own tree aside, and no check inside the lock can tell that from a first use.

These protections against another local user hold only when no other user can replace the lock's directory or any of its ancestors. That holds when each ancestor belongs to the user or root and is either not writable by others or sticky. 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.

Affected code

At the scanned commit b831496: _ensure_protocol_directory returns for any existing real directory, whatever its owner or mode. StrictSoftFileLock._acquire calls it for both levels and does not look at directory ownership again. _attempt_doorway decides from the claim scan alone, and _unlink_owner_path_result counts a missing claim as removed on release. The sibling site is OsFiles.prepare. Its mkdir(mode=0o700) takes effect only when it creates the directory, and its lstat check reads neither st_uid nor the permission bits. It runs once per instance, and every later OsFiles method takes a full path, with O_NOFOLLOW guarding only the last component.

Introduced

The strict site came with the claims protocol in 650c17c79c54 ("feat(strict): replace shared markers with owner claims", #659, 2026-07-14), first released in 3.30.0. The SoftReadWriteLock site came with the <path>.rw tree itself in f9743288a288 ("fix(soft-rw): replace the state mutex with a generation log", #738, 2026-09-17), first released in 4.0.0. Both match the git tags; the advisory range starts at the earlier one.

Mechanism

The owner of a directory can unlink and rename every entry in it, whatever the entries' own owner and mode. A strict hold consists of two names, intent-v1-<token>.claim and held-v1-<token>.claim, in <path>.filelock/claims. The sentinel (the lock file at <path>) is permanent and says nothing about who holds. When the other user runs the equivalent of mkdir -m 777 -p work.lock.filelock/claims before the lock user first uses work.lock, every lock user acquire publishes its claims into a direc