Valve Anti-Cheat (VAC)

Valve Anti-Cheat (VAC) is Valve's system for detecting cheats in games on Steam, introduced in 2002 with Counter-Strike. It runs on the client and scans memory for the signatures of known cheat software; find a match and the account gets flagged.

The signature trait that made VAC notorious is the delay: bans are often issued in waves weeks after detection, so cheat makers can't immediately tell which version got caught. A VAC ban is permanent, tied to the account, and locks you out of games in the same family. The system draws criticism in turn for missing new cheats and for false positives — the endless arms race between protection and circumvention.