CVE-2026-112908: filelock: StrictSoftFileLock creates its claim directories without the lock mode, so another local user with a write bit on them can end a live hold

StrictSoftFileLock and AsyncStrictSoftFileLock apply the caller’s mode to the sentinel (the lock file at <path>) and to both claim files (the files whose presence says a process wants or holds the lock). They create the two directories that hold the claims, <path>.filelock and <path>.filelock/claims, with a bare mkdir(). That gives 0o777 filtered through the umask (a per-process mask that removes permission bits from new files and directories) or, under a parent with a default ACL (an access control list on a directory that sets the permissions of entries created inside it), through that ACL.

A hold consists of two claim names in that directory, which has no sticky bit (a directory flag that lets only a file’s owner remove it). When the environment leaves another local user (the other user below) a write bit on the directory, that user can unlink the holder’s 0600 claims. Three settings do this: umask 000 in a sticky directory such as /tmp, umask 002 with the other user in the directory’s group, or a default ACL on the lock’s parent that grants the group write (the kernel then ignores the umask). After the unlink, the next contender of the holder’s account finds the directory empty and enters. The first holder keeps reporting is_locked == True and later releases with no error. Affected versions: filelock >= 3.30.0 and <= 4.2.0 (finding ANT-2026-AD6SUOON).