Tox-dev Filelock: UNIX Hard Link (CVE-2026-112934)

SoftReadWriteLock (and AsyncSoftReadWriteLock, which wraps one) refreshes its holder record (the file under <path>.rw/holders/ that says a process holds or wants the lock) on every poll of a waiting acquire and on every heartbeat (the periodic refresh a holder runs to keep its record fresh). It does so by reopening <path>.rw/holders/<token> by name with a plain blocking O_WRONLY | O_NOFOLLOW open and writing at offset 0, without checking what it opened. O_NOFOLLOW makes the open fail only when the final name is a symbolic link (a file that redirects to another path).

A different local user (the other user below) who owns the lock's .rw tree reads the token of the user running the lock (the lock user below) from the holders/ listing and renames another node over the record. The other user owns the tree because it created it first in a sticky directory such as /tmp (the sticky bit is a directory flag that lets only a file's owner rename or remove it), or because it can write a lock directory without the sticky bit.

A FIFO (a named pipe: a file whose open for writing blocks until some process opens it for reading) makes the lock user's write_lock() / read_lock() hang regardless of timeout. It also stops a lock holder's heartbeat thread without on_compromise (the callback that tells a holder it lost the lock) ever running, so the lock grants a second process while the first still believes it holds it. This works on a default Linux.

Where the kernel lets a user hard-link a file it does not own (a hard link is a second name for the same file; Linux with fs.protected_hardlinks=0; macOS and the BSDs by their defaults, not executed), a hard link makes the lock user overwrite the first ~135 bytes of one of its own private files with its holder record. Affected versions: filelock >= 4.0.0 and <= 4.2.0. Anthropic's OSS Scanner reported it in private as ANT-2026-AT33VEVC [medium, CWE-62], titled "SoftReadWriteLock holder refresh writes through hard links, hangs on FIFOs".

Severity

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H, 6.1 (Medium).

AV:L (local): the other user needs an account on the same host. AC:L (low attack complexity): on a default Linux there is no race. PR:L (low privileges): a local user with ordinary privileges is enough. UI:N (no user interaction): the lock user's process acts on its own. C:N (no confidentiality impact): the other user learns nothing. I:L (low integrity impact): where the kernel permits hard links to files one does not own, the lock user also writes its holder record over the first ~135 bytes of one of its private files. The bytes are filelock's record, not chosen by the other user, so the change stays within that record. A:H (high availability impact): the other user makes a lock user's acquire outlast any timeout and leaves a holder's heartbeat thread asleep. The lock then evicts the holder without telling it, and two processes believe they hold the write lock.

Classification from the external review: conditional security. The owner of a planted .rw tree can make the refresh open a FIFO and stall a lock user, including its heartbeat. Overwriting a file of the lock user through a hard link also needs a permissive hard-link policy. The scanner rated it medium.

Scope and preconditions

• Trust boundary: the other party is a different local account that owns the lock's .rw tree. Either it created <path>.rw (with gen/ and holders/) before the lock's first use in a sticky directory such as /tmp, where prepare() adopts an existing tree whoever owns it and whatever its mode. Or the lock lives in a directory without the sticky bit that the account can write (the scanner checked a root-created 0777 directory). There being first is not needed, because the other user renames the lock user's own 0700 tree aside and puts its own in place; prepare() does not run again for a live lock object, and a waiting lock user recreates its record in the replacement. A tree the lock user already created in a sticky directory (mode 0700) is out of reach.
• FIFO variant: needs only that boundary. It works on a default Linux. fs.protected_hardlinks plays no part, and fs.protected_fifos=1 (a Linux setting that refuses some opens of FIFOs in sticky world-writable directories) does not help, since it covers O_CREAT opens in sticky directories only, and the other user renames the FIFO into place inside its own holders/.
• Hard-link variant: also needs a kernel that permits a hard link to a file the caller does not own. That is Linux with fs.protected_hardlinks=0 (the kernel's built-in value; observed), while most distributions and the scanner's test image boot with 1, under which the other user's link() fails with EPERM (operation not permitted) and the file stays untouched. macOS and the BSDs have no such restriction by default; that follows from their link(2) semantics, and the scanner did not execute it. The target must be a regular file the lock user may write, on the same filesystem as the lock, in a directory the other user can search. A 0600 file in a 0755 home directory qualifies; a file under a 0700 directory does not.
• Platform: observed on Linux 6.1. AsyncSoftReadWriteLock runs the same code through an executor thread and loses that thread (from the code, not run). On Windows named pipes cannot live in a regular directory, so there is no FIFO case there.
• Ancestor boundary: protection against another local user is possible only while no other user can replace the lock's directory or any of its ancestors. That is the case when each ancestor belongs to the lock user or root and is either not writable by others or sticky.

Affected code

At the scanned commit b831496b0273, OsFiles.overwrite opens by name with O_WRONLY | O_NOFOLLOW and writes without fstat (fstat would report the type of the file behind the open descriptor), while read just above it uses O_NONBLOCK and a regular-file check. The blocking open is _attempt_open via _open. prepare adopts an existing tree whoever owns it. The single caller is Participant._refresh_nonce, reached from _keep_claim_fresh on every poll of a waiting acquire and from heartbeat, which knows only lost and transient, not a call that does not return. The acquire loop checks its deadline and the _closed flag only after advance() returns (_sync.py lines 584–592). The code maps the heartbeat outcome at lines 603–613, and release joins the heartbeat thread for heartbeat_interval + 1 at line 446.