Valve Anti-Cheat

Valve Anti-Cheat (VAC) is an automated anti-cheat system developed by Valve and first released with Counter-Strike in 2002 to enforce rules in online multiplayer games distributed through the Steam platform. The system detects unauthorized modifications to a game, including third-party software designed to manipulate gameplay, as well as alterations to core game files such as executable programs and dynamic-link libraries. Modifications that provide an unfair advantage are classified as cheats and are subject to detection by VAC.

When cheat software is detected on a user's system, VAC issues a permanent ban preventing access to VAC-secured servers for the specific game involved. Bans are applied automatically and may result from various factors, most commonly the detection of known cheat signatures. Enforcement may occur days or even weeks after the initial detection, depending on the game and circumstances.

Valve does not disclose to the player the date of detection, the type of cheat identified, or the specific method by which the cheat was detected. VAC bans are typically permanent and non-negotiable, although Valve retains the ability to remove them in certain cases, such as confirmed false positives, through internal review.

During one week of November 2006, the system detected over 10,000 cheating attempts, and during the month of December 2018 over 600,000 accounts were banned.

In 2001, Even Balance Inc., the developers of the anti-cheat software PunkBuster designed for Counter-Strike and Half-Life mods, stopped supporting the games as they had no support from Valve. Valve had also rejected business offers of integrating the technology directly into their games.

Valve started working on a "long-term solution" for cheating in 2001. VAC's initial release was with Counter-Strike in 2002. During this initial release, the system only banned players for 24 hours. The duration of the ban was increased over time; players were banned for 1 year and 5 years, until VAC2 was released in 2005, when any new bans became permanent. VAC2 was announced in February 2005 and began beta testing the following month. On November 17, 2006, they announced that "new [VAC] technology" had caught "over 10,000" cheating attempts in the preceding week alone.

During the early testing phase in 2002, some information was revealed about the program via the Half-Life Dedicated Server mailing lists. It can detect versions of "OGC's OpenGl Hack", can detect OpenGL cheats, and also detects CD key changers as cheats. Information on detected cheaters is sent to the ban list server on IP address 205.158.143.67 on port 27013, which was later changed to 27011. There is also a "master ban list" server. RAM/hardware errors detected by VAC may kick the player from the server, but not ban them.

Eric Smith and Nick Shaffner were the original contacts for game administrators. In February 2010, the VAC Team consisted of Steam's lead engineer John Cook and his team of 16 engineers.