CVE-2026-112909

filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.

UnixFileLock, the backend of FileLock and AsyncFileLock on Unix, empties whatever regular file it finds at the lock path once it holds the flock (a kernel lock on an open file). With an explicit mode it also chmods that file (sets its permission bits), whether or not this acquire created it. Another local user who can rename or hard-link one of the files of the user running the lock (below: the lock user) onto the lock path gets that file truncated by the lock user's next acquire(). A hard link is a second name for the same file. With a mode such as 0o666 the file, still owned by the lock user, also becomes readable and writable by that other user. The other user can then read and alter everything the lock user writes to it afterwards.

O_NOFOLLOW, the defence added for the symlink variant (CVE-2025-68146), does not see a renamed file or a hard link. The rename route needs the file's directory to be writable without the sticky bit (a directory flag that lets only a file's owner remove or rename it). The hard-link route into a sticky directory such as /tmp needs fs.protected_hardlinks=0, a Linux kernel setting. Affected versions are every filelock release up to and including 4.2.0 (the truncation is as old as the project; the chmod since 3.10.2).

Anthropic's OSS Scanner reported this in private. Finding covered by this advisory:

• ANT-2026-KO573NWI [high, CWE-706]: UnixFileLock truncates and chmods victim files renamed onto the lock path.

Severity

CVSS 3.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N scores 6.1 (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:L (low attack complexity): one rename or link, then wait for the lock user's next acquire; no race. I:H (high integrity impact): the lock user's own process truncates a file it did not mean to touch, including content behind a second name the other user could neither modify nor delete. With a permissive explicit mode the other user can then write to it. If that file is one the lock user later executes or trusts, that is code execution as the lock user. C:L (partial confidentiality impact): the old content is gone before the other user can read anything, but what the lock user writes afterwards through a descriptor (the number the kernel gives a process for an open file) it still holds is readable. A:N (no availability impact). S:U (scope unchanged).

Classification from the review: security, conditional on another user controlling names in a non-sticky directory on the lock's filesystem (or on fs.protected_hardlinks=0). The scanner rated this high. The maintainers score it medium because the reachable targets depend on the directory layout and kernel settings described below.

Scope and preconditions

• Lock classes: FileLock and AsyncFileLock on Unix (UnixFileLock). SoftFileLock applies an explicit mode only to a file it created with O_CREAT | O_EXCL and does not truncate an existing file. WindowsFileLock has not truncated at any version.
• The other user must put a regular file of the lock user at the lock path before the lock user's next acquire, while the lock file does not exist yet. That is the case on first use, after a reboot, after a /tmp cleaner, or after a release with the default preserve_lock_file=False, which unlinks the file. The scanner checked three ways at the scanned commit, with default kernel settings unless stated:
  • rename inside a directory without the sticky bit (a group-writable project or cache directory, a root-created 0777 directory);
  • rename from such a directory into a sticky one (1777) on the same filesystem. The sticky bit only protects entries that already exist, so the move succeeds while the lock path is free. Once the lock user's lock file exists there, the same move fails with EPERM;
  • link(2) into a sticky directory. This needs a…

filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.