5.3 Medium

filelock: SoftFileLock stale-break restore follows a swapped-in symbolic link and hard-links a file of the user running the lock into the lock directory

CVE-2026-112914

When a soft-lock stale break finds that the marker (the lock file at ) it renamed aside was a live one recreated by a successor, the restore branch of breaklockfile() checks the break name with lstat (a check that reads a name without following a symbolic link) and then calls os.link(breakpath, lockfile) by name. On CPython 3.12 and later on Linux (and on macOS) that call follows a symbolic link (a file that redirects to another path). A local user (the other user below) who can write a shared lock directory without the sticky bit (a directory flag that lets only a file's owner rename or remove it) replaces the break name with a symbolic link between those two calls. The SoftFileLock, MarkerSoftFileLock, SoftFileLease or lifetime break of the user running the lock (the lock user below) then creates a hard link (a second name for the same file), in that directory, to any of the lock user's own files on the same filesystem. That includes files inside directories the other user cannot enter and links that fs.protectedhardlinks (a Linux setting that forbids hard links to files one does not own) refuses the other user when it asks itself. The link exposes the file's size, times, mode…

Assessment

CVSS: 5.3 (v3.1)
Severity: Medium
EPSS: 0.00161% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 23 October 2026
Last updated: 23 October 2026
Affected: pip · filelock
Weakness: CWE-363, CWE-59

CVSS vectorCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:N

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