It's not that you have a user number, or even that it's accessible: it's that it's the entirety of access control, meaning that if the user changes it they can masquerade as another user. The correct solution is that a user should authenticate, which creates a session hash that you stash away and also send back to the user as a cookie so the browser will present it on accesses. Then you check that the presented hash is there and matches the session hash. These should expire periodically, requiring the user to log back in again.