Affected: tox-dev filelock

filelock is a platform-independent file lock for Python. From 3.32.6 until 4.2.1, SoftFileLock can follow a swapped symbolic link during the conditional os.access writability check for a node without its owner-write bit, allowing another local user on a platform and directory configuration that follows the link to infer whether a chosen path exists and is writable from the observable acquisition result. This issue is fixed in version 4.2.1.

Affected: PyPI: filelock

`raise_on_not_writable_file()` is the check that `SoftFileLock` and the locks sharing its acquire path run before every attempt. It looks at the lock path with `os.lstat`, so that it judges a planted symlink (a file that points at another path) by itself. When the node has no owner-write bit, it then calls `os.access(filename, os.W_OK)`, which follows symlinks. A local user who can create entries in the lock's directory can swap the path between a mode-`0466` file and a symlink to a path of their choosing. The second call then reports on the symlink's target with the permissions of the user running the lock (below: the lock user). The lock user's `acquire()` ends in `PermissionError` within a fraction of a second when the target is missing or not writable by the lock user, and in the ordinary `Timeout` otherwise.

That yields one bit per watched acquire. The bit says whether a path exists and is writable by the lock taker. It covers paths under directories the other user cannot search, or any path when root takes the lock. The lock does not open, create or modify anything through the symlink. On Linux, `fs.protected_symlinks` (a kernel setting that refuses to follow another user's symlink in a sticky world-writable directory) blocks it in a directory such as `/tmp`. It does not block it in a `0777` directory without the sticky bit (a directory flag that lets only a file's owner remove or rename it) or in a sticky group-writable one. This reopens the leak PR #567 closed. Affe…