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

Published

2026-10-28

Modified

2026-10-28

Aliases

GHSA-6627-m3xw-93mp

Sources

github-advisory

Summary

`StrictSoftFileLock` and `AsyncStrictSoftFileLock` apply the caller's `mode` to the sentinel (the lock file at ` `) 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, ` .filelock` and ` .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 .claim` makes the next `acquire()` raise `SoftFileLockProtocolError` ("malformed claim record") until someone removes it. Classification from the review: security, conditional on a lax umask or a permissive default ACL. The scanner rated it high (CWE-732). ## Scope and preconditions - The holder passes a restrictive `mode` such as `0o600`. Without one, the umask decides the permissions by documented design (the "File permissions and mode" section of `docs/concepts.rst` defines `mode` only for the lock file). Umask `000` with the default mode gives the same two-holder result but is outside this finding. - The holder's environment leaves the other user a write bit on new directories. The scanner measured three ways, with `mode=0o600` and the other user a member of group `team`: | lock's parent directory | holder's umask | ` .filelock` and `claims/` | who can unlink the claims | |---|---|---|---| | `1777`, like `/tmp` | `000` | `0777` | any other local user | | `root:team`, `3775` (setgid, so new entries inherit the group, and sticky) | `002` | `2775` | any member of `team` | | `root:team`, `1770`, default ACL `group:team:rwx` (`setfacl -d -m g:team:rwx`) | `077` | `0770` | any member of `team` | | `root:team`, `3775`, no ACL | `022` | `2755` | no one; the second process times out | The default-ACL row is the strongest. A holder with a private umask and `mode=0o600` still gets group-writable claim directories. The lock file in that row is `0600`, because its mode reaches `os.open` and masks the inherited ACL. - A contender that may enter asks for the

Affected packages

Remediation: Upgrade to 4.2.1 or later.

References

• https://github.com/advisories/GHSA-6627-m3xw-93mp