CVE-2026-112935

filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can break a stale marker after an attacker frees its own marker following the stale read, an honest contender creates a live successor on the reused inode, and the successor's modification time is not newer than the saved observation, causing the live marker to be removed and two holders to be admitted. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can break a stale marker after an attacker frees its own marker following the stale read, an honest contender creates a live successor on the reused inode, and the successor's modification time is not newer than the saved observation, causing the live marker to be removed and two holders to be admitted. This issue is fixed in version 4.2.1.

The stale-lock break shared by SoftFileLock, MarkerSoftFileLock and SoftFileLease (and their async variants, and a FileLock that has fallen back to a soft lock on ENOSYS, the "not implemented" error) recognises the marker it judged stale by two values. The marker is the lock file at <path>, whose presence says a process holds the lock. The values are its inode number (the kernel's identity for a file, independent of its name) and its modification time (mtime) not being newer. The check runs after the contender has already closed the marker, so nothing keeps the inode allocated.

On a filesystem that hands a freed inode number straight to the next file (ext4, xfs, overlayfs on them) the next holder's marker can carry that number. A contender still between its read and its break then renames and unlinks the live marker. Another local user (the other user below) who can create files in the lock's directory, a sticky /tmp included (the sticky bit is a directory flag that lets only a file's owner remove it), forces this at will. It plants a marker with a stale owner record and a modification time in the future and unlinks it as soon as a contender has read it. Two processes of the user running the lock (the lock user below) then hold the same default-configured lock at once, and nothing raises or logs. Affected versions: filelock >= 3.22.0 and <= 4.2.0 (finding ANT-2026-KIIS96IX).

Severity

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N, 4.7 (Medium).

AV:L (local): the other user needs an account on the same host. AC:H (high attack complexity): a race, though one the other user starts, lengthens (by making the lock user's /proc read slow) and retries at every release. The scanner measured 10 to 26 overlaps per 20-second run. PR:L (low privileges): an unprivileged account is enough. UI:N (no user interaction): the lock user's processes act on their own. C:N (no confidentiality impact): the other user reads nothing. I:H (high integrity impact): two holders write the data the lock protects, and neither learns of the other. A:N (no availability impact): the lock keeps working.

Classification from the review: security, conditional on a sticky shared directory, ext4-style inode reuse and at least two contenders of the lock user. Our review verified it across UIDs (a UID is a user account's numeric id): the other user as uid 1001, the lock user's processes as uid 1000, BUG on 4.2.0 over ext4. The scanner rated it high (CWE-367).

Scope and preconditions

• The other user can create files in the lock's directory; a root-owned 1777 directory such as /tmp or /var/tmp is enough. It creates, utimes (sets the timestamps of) and unlinks only its own file. The sticky bit stops it removing the lock user's marker (checked: EPERM, operation not permitted), so the lock user's own stale break does the deletion.
• The directory is on a filesystem that hands a freed inode number straight to the next file. ext4 does (six create/unlink rounds returned one number six times, in the image's overlayfs /tmp and on a loop-mounted ext4), and xfs does. tmpfs returned six increasing numbers and the same run there gave no overlap, and btrfs does not hand a freed number back.
• The lock user has at least two processes contending for the lock, and the displaced holder is still inside when the next contender polls (the breaker itself comes back one poll_interval, 50 ms by default, later).
• The lock is SoftFileLock, MarkerSoftFileLock or SoftFileLease (or async forms). The default flock-based FileLock joins them only after it has fallen back to SoftFileLock on ENOSYS. StrictSoftFileLock does not call break_lock_file, so it is not affected.
• The same overlap also occurs with no other user involved, under heavy same-account contention, when a contender breaks a dead holder's marker and a successor creates its marker within the same timestamp tick (about once in 750 crashes with 64 workers on a…

filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can break a stale marker after an attacker frees its own marker following the stale read, an honest contender creates a live successor on the reused inode, and the successor's modification time is not newer than the saved observation, causing the live marker to be removed and two holders to be admitted. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can break a stale marker after an attacker frees its own marker following the stale read, an honest contender creates a live successor on the reused inode, and the successor's modification time is not newer than the saved observation, causing the live marker to be removed and two holders to be admitted. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can break a stale marker after an attacker frees its own marker following the stale read, an honest contender creates a live successor on the reused inode, and the successor's modification time is not newer than the saved observation, causing the live marker to be removed and two holders to be admitted. This issue is fixed in version 4.2.1.