|Operating system||Windows, macOS, Linux|
When the software detects a cheat on a player's system, it will ban them in the future, possibly days or weeks after the original detection. It may kick players from the game if it detects errors in their system's memory or hardware. No information such as date of detection or type of cheat detected is disclosed to the player. After the player is notified, access to online "VAC protected" servers of the game the player cheated in is permanently revoked and additional restrictions are applied to the player's Steam account.
During one week of November 2006, the system detected over 10,000 cheating attempts.
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," OpenGL cheats, and also detects CD key changers as cheats. Information on detected cheaters is sent to the ban list server on IP address 126.96.36.199 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.
In July 2010, several players who successfully used information leaked from Valve to increase their chances of finding a rare Team Fortress 2 weapon/tool called the Golden Wrench found themselves banned by VAC. During the same month, approximately 12,000 owners of Call of Duty: Modern Warfare 2 were banned when Steam updated a DLL file on-disk after it had been loaded into memory by the game, causing a false positive detection. These bans were revoked and those affected received a free copy of Left 4 Dead 2 or an extra copy to send as a gift.
In February 2014, rumors spread that the system was monitoring websites users had visited by accessing their DNS cache. Gabe Newell responded via Reddit, clarifying that the purpose of the check was to act as a secondary counter-measure to detect kernel level cheats, and that it affected one tenth of one percent of clients checked which resulted in 570 bans.
As of May 2016, the system began banning accounts that were registered with the same phone number. Additionally, a phone number that was used on an account at the time it was banned will not be allowed to be re-registered on other accounts for three months.
In February 2017, Valve announced plans to introduce a machine-learning approach to detecting cheats in Counter-Strike: Global Offensive, and that an initial version of the system was already in place, which would automatically mark players for manual detection by players through the "Overwatch" system. 
The software sends client challenges to the machine; if the appropriate response is not received, it is flagged as a possible violation. It uses Signature Scanning to detect possible cheats when scanning the computer's memory and processes. Whenever an anomaly is detected, an incident report is created and compared to a database of banned applications and/or analyzed by Valve engineers. The engineers may inspect the code and run it on their own copies of the game. If the code is confirmed as a new cheat, it is added to the database of cheat codes.
According to Steam's lead engineer John Cook, to stop the anti-cheat software itself from being exploited, "The software is constantly updated and sent down in small portions for the servers as needed, so hackers only get to see small portions of it running at any particular time. So while they may be able to work around pieces of it, they can never hack everything."
Valve also accepts submissions of cheat programs and cheat websites from players by email. Players may also report players they suspect of cheating through their Steam Community profile, although players are not banned from these reports alone.
If a cheat is found, the player's Steam account will be flagged as cheating immediately, but the player will not receive any indication of the detection. It is only after a delay of "days or even weeks" that the account is permanently banned from "VAC Secure" servers for that game, possibly along with other games that use the same engine (e.g. Valve's Source games, GoldSrc games, Unreal engine games). Valve never discloses which cheat was detected. Players have criticised the system for taking weeks to months to ban cheaters.
Players that are banned face additional restrictions. Steam Family Sharing allows users to share their video game library with another Steam user to download and play, but games that the player is VAC banned from cannot be shared. If a user shares their games with another user, then cheats or fraud are detected on the recipients account, the original owner of the games being shared may be VAC banned and the sharing function revoked. Banned users can also not contribute to the Steam Translation Server project, that allows users to contribute new translations of Steam and its games. Users banned from a game are not allowed to refund it.
Over 100 games support VAC, players that are banned from the following games face additional restrictions:
- Call of Duty: Modern Warfare 2
- Call of Duty: Modern Warfare 3
- Counter-Strike: Condition Zero
- Counter-Strike: Source
- Counter-Strike: Global Offensive
- Day of Defeat
- Day of Defeat: Source
- Deathmatch Classic
- Half-Life 2: Deathmatch
- Half-Life Deathmatch: Source
- Team Fortress 2
- Team Fortress Classic
Mods based on the games above may inherit VAC support from the host game.
Denotes GoldSrc games, if a player is banned in one of these games they are banned from all of them.
Denotes Source engine games, if a player is banned in one of these games they are banned from all of them.
Denotes games that have a stricter policy of having all servers VAC protected, and additionally bans players for editing of any game files except config files.
Denotes games that face digital goods restrictions and or revocation.
The user's Steam profile is also marked with "ban(s) on record", which is publicly visible and cannot be hidden, regardless of the profile visibility of the banned account. VAC ban's become hidden to other users after seven years of not getting another VAC ban. An analysis of 43,465 users that had been banned between April 2011 and October 2011 showed that the more VAC banned players a user is friends with, the more likely they will also be VAC banned themselves in the future. After they were banned, they lost more friends, were more likely to increase their privacy settings and also had more VAC banned friends than non-banned players. Banned players are also sometimes referred to as going on "VACation".
Banned players are also excluded from competing in most electronic sports tournaments. In 2014, professional player Joel "Emilio" Mako was banned during a live stream, he initially denied using a cheat, claiming it was caused by "a friend of his played on one of his smurfing accounts which mail is linked to his main account" Then in 2015, he admitted to using a cheat. Hovik "KQLY" Tovmassian, Simon "smn" Beck and Gordon "SF" Giry were banned shortly before they were scheduled to play at DreamHack Winter 2014. The ESEA League claimed the bans were a result of working with Valve directly. Simon "smn" Beck and Hovik "KQLY" Tovmassian both admitted to using cheats.
- "I've Been Banned". Valve Support FAQ. January 23, 2008. Retrieved September 18, 2008.
- "Steam Message". Steam Update News. November 17, 2006. Retrieved December 11, 2002.
- "Punkbuster to stop HL/CS development".
- "Punkbuster Retires From HL".
- "Eric Smith, Valve, HLDS mailing list, October 2001".
- "Online cheaters face games ban". BBC News. August 29, 2002. Retrieved August 23, 2006.
- "VACBanned - Information".
- "Steam - New Counter-Strike: Source Map Coming Soon". Steampowered.com. February 11, 2005. Retrieved July 12, 2014.
- "Steam - Update". Steampowered.com. March 25, 2005. Retrieved July 12, 2014.
- "Eric Smith, Valve HLDS Mailing List, July 2002".
- Eric Smith, Valve, HLDS mailing list, August 2002.
- "Eric Smith, Valve, HLDS mailing list, February 2003".
- "Eric Smith, Valve, June 2003".
- "CS-Nation". Archived from the original on June 19, 2003.
- "CS-Nation-the future of Counter-Strike". Archived from the original on November 18, 2003.
- "TF2 Engineer Update Gets Serious – VAC Bans Issued". DeviceMAG.
- "Golden Wrench Scandal – Team Fortress 2 Golden Wrench Stolen". Top Tier Tactics.
- Smith, Quintin. "Valve Anti-Cheat software goes a bit GlaDOS?". Rock, Paper, Shotgun. Retrieved July 26, 2010.
- "Valve offers free game after 12,000 false Steam bans". Gamesindustry.biz. GamesIndustry International. July 27, 2010.
- Kubba, Sinan (September 21, 2012). "Joystiq". Gamedaily.com.
- "Valve, VAC, and trust". Reddit.
- Warr, Philippa (February 18, 2014). "Valve couldn't care less what porn you watch (Wired UK)". Wired.co.uk. Archived from the original on May 8, 2014. Retrieved July 12, 2014.
- Chris Scullion Games Editor, CVG UK Follow @scully1888 (February 18, 2014). "Valve rejects claims it receives players' browsing history - CVG US". Computerandvideogames.com. Retrieved July 12, 2014.
- Prescott, Shaun. "Valve wants to take a 'machine learning' approach to Counter-Strike anti-cheat". PCGamer. PCGamer. Retrieved February 27, 2017.
- Autonomic and Trusted Computing: 4th International Conference, ATC 2007, Hong Kong, China, July 11–13, 2007. p125
- "Steamed: Valve Software Battles Video-game Cheaters".
- "Valve Anti-Cheat System (VAC)". Steam Support. November 15, 2006. Retrieved December 23, 2006.
- "Rust's new anti-cheat system is working - but Facepunch are "never going to be finished fighting"". PCGamesN. Retrieved July 12, 2014.
- Ms. Smith (February 16, 2014). "Some gamers steamed over alleged Valve anti-cheat DNS spying". Network World.
- Nathan Grayson. "A Lot Of Cheaters Just Got Banned From Counter-Strike". kotaku.com.au.
- "Steam Family Sharing". Valve Corporation. Retrieved July 5, 2014.
- Maiberg, Emanuel. "Steam Family Sharing now available for all users". PC Gamer. Retrieved July 5, 2014.
- "Steam Translation Server - Welcome". Translation.steampowered.com. Retrieved July 12, 2014.
- "Steam Refunds". Retrieved June 3, 2015.
- "Steam games with VAC".
- "Items Not Properly Awarded - Trading and Gifting - Knowledge Base - Steam Support". Support.steampowered.com. Retrieved July 12, 2014.
- Blackburn, Jeremy; Simha, Ramanuja; Kourtellis, Nicolas; Zuo, Xiang; Ripeanu, Matei; Skvoretz, John; Iamnitchi, Adriana (2012). Branded with a scarlet "C": cheaters in a gaming social network. International World Wide Web Conference. pp. 81, 89. doi:10.1145/2187836.2187848. ISBN 9781450312295. Retrieved October 12, 2014.
- Higgins, Chris (November 25, 2014). "How to spot a hacker in Counter-Strike". Redbull. Retrieved February 27, 2015.
- "DreamHack: "We will take special precautions at DHW"". E-sport.
- "Team Property statement regarding emilio". E-sport.
- "CS:GO eSports Community Shaken Following Revelation of Cheating". IGN.
- "HLTV.org - News: ESEA: "More to come soon"". HLTV.org.