On Windows the soft locks ("SoftFileLock", "MarkerSoftFileLock", "SoftFileLease", their async forms, and a "FileLock" that fell back to a soft lock) read their marker by path. The marker is the lock file whose presence says a process holds the lock. Each "acquire()" attempt that finds a marker opens it with "os.open" to read the holder's process id, and the lock renames and deletes a marker it judges stale, by path. On Windows each of these calls waits for an oplock. An oplock (opportunistic lock) is a request by a process that holds a handle on a file to be told before another process opens, renames or deletes that file; the other process waits until the holder answers the break or closes its handle. Another local account that can open the marker for reading can take a Level 1 oplock on it while no process holds it open, and leave the break unanswered. Every "acquire()" by the user running the lock (below: the lock user) then waits inside the open, past its "timeout", "blocking=False" and "cancel_check", and a stale marker under that oplock is not broken. Mutual exclusion holds and the lock touches no other file. Affected versions are filelock 3.22.0 through 4.2.0 on Windows. The maintainers found this while reviewing the fix for the report Anthropic's OSS Scanner sent on 15 October 2026 (review round 4, finding F4); it is not among the scanner's findings.