FSAC: FileSystem Anti-Cheat
Cheating is a prolific problem in Team Fortress 2, as Vavle's own anti-cheat software is entirely bypassed by modern hacks. FSAC is a server-side plugin to prevent cheaters from generating new accounts and returning to continue cheating.
When a server administrator bans a player using FSAC, the plugin injects file fragments to their game's file structure, disguised as important files, in hidden locations.
The server checks all clients upon joining for any of these fragments. If any of the files are detected, the player is re-banned, all files are redelivered for integrity, and the account's ID is recorded.
All FSAC bans, server-detected or admin-committed, are recorded in MySQL for recording purposes, as FSAC is potentially very dangerous.