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Many Source engine games are run on servers that are "VAC-Secured". How does the VAC really work? What is it looking for? If it finds something "bad", what does it do?

Is there any way a user can appeal a VAC ban?

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    I ask this question not so that I can get around the VAC, but just so I don't make some mod that I should not be making. And, if I were to get "caught", what I can do about it.
    – Jeffrey
    Commented Jul 27, 2010 at 13:12

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From the Steam KB:

Important:

All VAC bans are permanent - Valve has a zero-tolerance policy for cheating and will not lift VAC bans under any circumstances.

Also,

Any third-party modification to a game designed to give one player an advantage over another is classified as a cheat or hack and will trigger a VAC ban. This includes modifications to a game's core executable files and dynamic link libraries.

And

The following will not trigger a VAC ban:

  • Using chat programs like X-Fire
  • Playing multi-player mods which do not modify core .EXE and .DLL files
  • System hardware configurations
  • Updated system drivers, such as video card drivers
  • Using custom skins, sounds or map

So if your mod fall into second category but not first, you should be okay :)

It might also be good to know that VAC bans don't happen instantly, usually a random time between a week - 2 months you will see this ban occur. This is to help stop the cheaters from knowing which hacks actually get pass VAC Security.

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    That's an interesting line to walk. Multi-player mods are allowed, but not one that would "give one player an advantage over another".
    – Jeffrey
    Commented Jul 27, 2010 at 13:20
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    I assume it just means mods to multi-player games, like custom HUDs. I know lots of people use these, but you could argue that they may give an "advantage" to the players that use them.
    – Jeffrey
    Commented Jul 27, 2010 at 13:29
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    Jeffrey, If you note the wording in the 2nd quote, it says "third-party modifications that are designed to give one player an advantage over another." By that wording you coul dsee why a custom HUD might be legal, since they arent necessarily designed to be advantageous, just designed for specific user need...Just another thought I had
    – Josh K
    Commented Jul 28, 2010 at 12:20
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    There used to be a really obnoxious graphics pack for TFC that turned certain walls semi-transparent so you could see who was hiding behind them, extended long thick lines from the tip of a sniper's rifle so you could always see where one was hiding, and other strictly client-side graphics changes to make things much easier on the user -- until other players started asking how their snipers kept getting headshotted while in near-perfect cover, etc...
    – Shadur-don't-feed-the-AI
    Commented Mar 3, 2013 at 11:15
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    This clause has been removed from steam VAC page "Playing multi-player mods which do not modify core .EXE and .DLL files"
    – Abdul Rauf
    Commented Oct 14, 2015 at 19:53
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VAC works by scanning your system while you play any VAC supported game (TF2, L4D1+2, etc). I don't know what exactly it scans for, and in fact, that is probably not public knowledge at any rate.

Bans are unappeal-able, and final.

When it kicks in, you are unable to play online multiplayer on VAC secured servers.

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  • That last part isnt 100% true, they (your friends) can choose to play in a non VAC Secure server with you.
    – Josh K
    Commented Jul 27, 2010 at 13:43
  • Fixed if for you.
    – badp
    Commented Jul 27, 2010 at 14:41
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See here:

The VAC system reliably detects cheats using their cheat signatures. Any third-party modification to a game designed to give one player an advantage over another is classified as a cheat or hack and will trigger a VAC ban. This includes modifications to a game's core executable files and dynamic link libraries.

The following will not trigger a VAC ban:

  • Using chat programs like X-Fire
  • Playing multi-player mods which do not modify core .EXE and .DLL files
  • System hardware configurations
  • Updated system drivers, such as video card drivers
  • Using custom skins, sounds or maps

Use only trusted machines to play on VAC-Secured servers - if you are not sure whether or not the machine you are using to connect to Steam may have cheats installed, do not play on VAC-Secured servers.

Use caution when installing any game modifications, such as scripts or custom skins. Only download custom content from trusted sources. Hackers sometimes maliciously disguise their cheats to cause others to get banned.

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For copyright issues, we can't copy paste text here. Cough, cough.

Here are the relevant links:

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