Valve Anti-Cheat

Valve Anti-Cheat, more commonly known by its acronym VAC, is a proprietary anti-cheat system developed by Valve for use in both GoldSrc and Source multiplayer games.

Heuristics

When a player is connected to a VAC-secured server (denoted by a security badge and the letter V in the server browser), the VAC system checks if any foreign processes are hooked into the player's local game binaries. If the VAC check finds a positive ID for any possible cheating tool, the offending player's Steam account is then permanently banned from all VAC-secured servers after a variable amount of time.

Message displayed to a VAC-banned client.

Valve Anti-Cheat will not detect content hacks such as invisible wall textures or bright-colored player models. Server admins that wish to block such activity will need to run a pure server.

Using VAC

Valve Anti-Cheat is a shared element of the Source engine; thus, it is supported by any Source-based multiplayer game.

For server admins that wish to use VAC, you should run your server with the -secure command line parameter. If you are using the Steam srcds frontend, make sure that the Secure (Valve Anti-Cheat) option is checked.