filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

Affected: PyPI: filelock

`ReadWriteLock` and `AsyncReadWriteLock` (the SQLite-backed reader/writer lock) hand SQLite a name they do not control. On Linux the lock validates its database through a descriptor (the number the kernel gives a process for an open file). It then connects SQLite to `/proc/self/fd/<n>`, a Linux path that names the process's own open file `n`. SQLite turns that path back into a plain path and reopens it by name, following symlinks (a symlink is a file that points at another path). On every POSIX route SQLite also acts on `-journal`, `-wal` and `-shm` files it finds beside that name. Those are SQLite's side files: the rollback journal (pages saved to undo an interrupted write), the write-ahead log or WAL (changes kept outside the main file), and the shared-memory index for the WAL.

Another local user who can create or rename entries in the directory that holds the lock database can turn this against the user running the lock (below: the lock user). Such a directory is a `0777` or group-writable cache, build or project directory without the sticky bit (a directory flag that lets only a file's owner remove or rename it). That other user can make the lock user's ordinary `read_lock()` or `write_lock()` write bytes of the other user's choosing into the lock user's own files. The same position lets the other user make that call create or unlink files of the lock user, admit a reader beside a writer, or kill the holder. All of this runs with the lock user's privileges. In a sticky…

Affected: Ubuntu:Pro:18.04:LTS: python-filelock, Ubuntu:Pro:20.04:LTS: python-filelock, Ubuntu:Pro:22.04:LTS: python-filelock, Ubuntu:Pro:24.04:LTS: python-filelock, Ubuntu:26.10: python-filelock, Ubuntu:26.04:LTS: python-filelock

(filelock is a platform-independent file lock for Python. From 3.20.4 u ...)