TL;DR
Ask Cursor to fix a path traversal bug and you get path.basename, path.resolve, and a startsWith containment check. It looks like the textbook answer.
The check is pure string math. It never touches the disk, so it cannot tell that a file in your upload directory is a symlink pointing at /etc/passwd.
Use fs.realpath for the containment check so the comparison runs against where the file actually is, and stop accepting filenames from users at all.
I had a file download endpoint with a real path traversal bug in it. Textbook version: req.query.file went into path.join and out the other side into sendFile. I pasted it into Cursor and asked it to fix the traversal.
