CVE-2026-112906

filelock is a platform-independent file lock for Python. From 4.1.0 until 4.2.1, SoftFileLock versions from 4.1.0 can reuse a stale observation when an attacker pre-dates and removes its own marker, at least two lock-user processes are waiting, coarse or matching modification times preserve the observation, and an honest contender creates a fresh marker on the reused inode, causing that live marker to be removed as stale. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 4.1.0 until 4.2.1, SoftFileLock versions from 4.1.0 can reuse a stale observation when an attacker pre-dates and removes its own marker, at least two lock-user processes are waiting, coarse or matching modification times preserve the observation, and an honest contender creates a fresh marker on the reused inode, causing that live marker to be removed as stale. This issue is fixed in version 4.2.1.

SoftFileLock and MarkerSoftFileLock evict an unparsable marker (the lock file at <path>, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. SoftFileLease expires a lease the same way. The watch tells two looks apart by the marker's modification time (mtime) and inode number (the kernel's identity for a file, independent of its name) alone.

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), parks an empty file of its own at the lock path. It dates that file to a future tick of the kernel's coarse clock (a clock that advances in steps of 4 ms), lets the waiters of the user running the lock (the lock user below) watch it past two seconds, and unlinks it in that tick. On a filesystem that hands a freed inode number straight back (ext4, overlayfs on ext4) the lock user's next marker repeats both values. It inherits the time spent watching the other user's file, and a second waiter removes it while its owner holds the lock.

Two of the lock user's processes then run the critical section together with no error. A SoftFileLease waiter takes a live lease that its owner wrote moments before, within milliseconds instead of after lease_duration. Affected versions: filelock >= 4.1.0 and <= 4.2.0 (finding ANT-2026-RKC48J5B).

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 the other user cannot steer. It needs ext4-style inode reuse, a coarse-clock timestamp tick the other user can predict, and two polls of the lock user inside a 4 ms tick and a ~75 µs create-to-write gap. The scanner measured 2 to 10 % of attempts; none of our own 250 attempts fired. 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, by mechanism. It is the same cross-UID primitive (a UID is a user account's numeric id) as the stale-break inode-reuse race reported in a separate advisory: an inode number reused after the other user's unlink in a sticky directory, which our review reproduced across UIDs, plus a forged future mtime. The scanner rated it high (CWE-694) and demonstrated it across UIDs in its own runs. It did not fire in 250 attempts of ours over ext4 with inode reuse confirmed, nor in the scanner-side triage, so its cross-user claim rests on the report's own runs and on the mechanism.

Scope and preconditions

• The other user can create files in the lock's directory; a root-owned 1777 directory is enough. It creates, utimes (sets the timestamps of) and unlinks only its own file. In the sticky directory its unlink, rename, open(..., "w") and utime on a marker of the lock user all fail, and deleting its own file only frees the lock.
• The directory is on a filesystem that reuses a freed inode number at once: ext4, and overlayfs on it (thousands of create/unlink cycles returned one number). tmpfs does not reuse one, so the pair cannot repeat there; btrfs does not either.
• File timestamps come from a coarse clock whose future values the other user can compute (CLOCK_REALTIME_COARSE, 4 ms steps on Linux 6.1; 20,000 file creations all carried that clock's value).
• At least two processes of the lock user are waiting on the lock while the other user's file is there. The other user does not choose which processes overlap or when; each attempt blocks the lock for 2.2 s (for a lease, lease_duration).
• No symbolic link, hard link or FIFO…

filelock is a platform-independent file lock for Python. From 4.1.0 until 4.2.1, SoftFileLock versions from 4.1.0 can reuse a stale observation when an attacker pre-dates and removes its own marker, at least two lock-user processes are waiting, coarse or matching modification times preserve the observation, and an honest contender creates a fresh marker on the reused inode, causing that live marker to be removed as stale. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 4.1.0 until 4.2.1, SoftFileLock versions from 4.1.0 can reuse a stale observation when an attacker pre-dates and removes its own marker, at least two lock-user processes are waiting, coarse or matching modification times preserve the observation, and an honest contender creates a fresh marker on the reused inode, causing that live marker to be removed as stale. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 4.1.0 until 4.2.1, SoftFileLock versions from 4.1.0 can reuse a stale observation when an attacker pre-dates and removes its own marker, at least two lock-user processes are waiting, coarse or matching modification times preserve the observation, and an honest contender creates a fresh marker on the reused inode, causing that live marker to be removed as stale. This issue is fixed in version 4.2.1.

SoftFileLock and MarkerSoftFileLock evict an unparsable marker (the lock file at <path>, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. SoftFileLease expires a lease the same way. The watch tells two looks apart by the marker's modification time (mtime) and inode number (the kernel's identity for a file, independent of its name) alone.

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), parks an empty file of its own at the lock path. It dates that file to a future tick of the kernel's coarse clock (a clock that advances in steps of 4 ms), lets the waiters of the user running the lock (the lock user below) watch it past two seconds, and unlinks it in that tick. On a filesystem that hands a freed inode number straight back (ext4, overlayfs on ext4) the lock user's next marker repeats both values. It inherits the time spent watching the other user's file, and a second waiter removes it while its owner holds the lock.

Two of the lock user's processes then run the critical section together with no error. A SoftFileLease waiter takes a live lease that its owner wrote moments before, within milliseconds instead of after lease_duration. Affected versions: filelock >= 4.1.0 and <= 4.2.0 (finding ANT-2026-RKC48J5B).

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 the other user cannot steer. It needs ext4-style inode reuse, a coarse-clock timestamp tick the other user can predict, and two polls of the lock user inside a 4 ms tick and a ~75 µs create-to-write gap. The scanner measured 2 to 10 % of attempts; none of our own 250 attempts fired. 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, by mechanism. It is the same cross-UID primitive (a UID is a user account's numeric id) as the stale-break inode-reuse race reported in a separate advisory: an inode number reused after the other user's unlink in a sticky directory, which our review reproduced across UIDs, plus a forged future mtime. The scanner rated it high (CWE-694) and demonstrated it across UIDs in its own runs. It did not fire in 250 attempts of ours over ext4 with inode reuse confirmed, nor in the scanner-side triage, so its cross-user claim rests on the report's own runs and on the mechanism.

Scope and preconditions

• The other user can create files in the lock's directory; a root-owned 1777 directory is enough. It creates, utimes (sets the timestamps of) and unlinks only its own file. In the sticky directory its unlink, rename, open(..., "w") and utime on a marker of the lock user all fail, and deleting its own file only frees the lock.
• The directory is on a filesystem that reuses a freed inode number at once: ext4, and overlayfs on it (thousands of create/unlink cycles returned one number). tmpfs does not reuse one, so the pair cannot repeat there; btrfs does not either.
• File timestamps come from a coarse clock whose future values the other user can compute (CLOCK_REALTIME_COARSE, 4 ms steps on Linux 6.1; 20,000 file creations all carried that clock's value).
• At least two processes of the lock user are waiting on the lock while the other user's file is there. The other user does not choose which processes overlap or when; each attempt blocks the lock for 2.2 s (for a lease, lease_duration).
• No symbolic link, hard link or FIFO…

filelock is a platform-independent file lock for Python. From 4.1.0 until 4.2.1, SoftFileLock versions from 4.1.0 can reuse a stale observation when an attacker pre-dates and removes its own marker, at least two lock-user processes are waiting, coarse or matching modification times preserve the observation, and an honest contender creates a fresh marker on the reused inode, causing that live marker to be removed as stale. This issue is fixed in version 4.2.1.