Valve Anti-Cheat
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Valve Anti-Cheat, abbreviated as VAC, is an anti-cheat solution developed by Valve Corporation as a component of the Steam digital game distribution platform.
Functionality
VAC-bans are engine-wide, applying to all games running on a single engine. For example, users that are banned on Counter-Strike or Half-Life will be banned on all games running the GoldSource engine. However, they can still play games and connect to secure servers on games that do not use the same engine. VAC-banned players are still able to play on all VAC-insecure servers on any game.
In the past, those who were VAC Banned had all non-purchased items deleted from their backpack. In the current version of Team Fortress 2, users who are VAC banned do not lose any items in their backpack. Instead, all items they have are stuck inside of their backpack and cannot be traded away, placed on the Steam Community Market, or transferred to other accounts in any way. This is done to prevent players from trading the items to another account and cheating again. These users are still able to purchase items off of the Mann Co. Store, and use said items freely.
VAC bans are non-negotiable. With few exceptions that may occur due to account theft of very rare false positives. VAC targets cheats that tamper with the client, it does not secure against vulnerabilities server-side.
Effectiveness Against Bots
The delayed ban system of VAC has proven to be ineffective against bots, which can be replaced quickly by their hosts, making bans effectively worthless when they happen. Valve has recognized this, and they have adopted an approach of combating vulnerabilities inside the TF2 client itself, instead of focusing on the VAC system.
VAC Functionality on GNU/Linux
Contrary to several rumors originated by the fact that TF2 botting software is Linux-exclusive, the versions of VAC on Linux, Mac and Windows enjoy a level of complete parity. This means each version of VAC works the same, independently of the platform it's running on. This parity originates from VAC being designed as client-level only, meaning that it can be easily maintained on various operating systems without harming security or infringing on the user's privacy by installing a rootkit.
