VAC Authentication Reliable, Permanent Solution For All Situations:

[Commander Makara]

The "VAC authentication error" or "A problem with your computer is blocking the VAC system" is somethat vague in description, but that is because it stems from the fact that telemetry readings failed at some point between the sending of the test and the receipt of response.
There is no way for Valve to know exactly what occurred, only that the expected response did not arrive.

"But I haven't changed anything, and it used to work fine, why is it giving me this error now? Surely Valve did something!"
Computer Operating systems are complex and (some would say bloated) delicate. You may not be aware of the changes you are making on a daily basis simply by opening a file/folder or running a program - this generates and alters many areas from log files, caches, registry entriess, temporary files, NT data stream access records and more. Also, certain 3rd party 'security' software may not be explicit in the actual effects provided by certain protection methods they may employ, such as modifying the Policy settings etc.
Regardless, even if Valve have changed something, they would stiill have tested it on clean machines, so this would only cause the error to be apparent on a machine on which the original configuration has been changed historically.
In short, you might change something today and not notice the error until that change necessarily becomes critical to the operaiton of VAC.

"But Why are Valve doing this, we're honest players and being punished!"
Unfortunately, like so many such cases, those devious cheat developers resort to ever more and more technical methods to disguise/hide their cheats from detection. Since a significant proportion of the playerbase population routinely complain if they see but a single cheat in a game, and by Valve's own 'zero-tolerance' policies, VAC is then required to scan in a deeper and more complex way. This necessitates access to certain files and is supported by the features within Windows itself to secure low-level layers.
If it weren't for those cheat developers at all and public demand for a solution, there would be no VAC at all.

The potential reasons for this occurrence are too numerous to name, but may be classified in 4 distinct groups:

i) A requested file could not be accessed. This may be due to:
◾ Being locked while in use by another process
◾ Access prevented by security permission policy or anti-malware (especially Live or RealTime-Protection modes) system
◾ The file is missing or corrupt.
ii) Connectivity issues. These may be:
◾ Bad conenction.
◾ Incorrectly configured Firewall, disabled / 'emergency mode' Firewall
◾ Anti-Malware system preventing network activity to a required process
◾ Incorrect port configurations for Router/Gateway
iii) Conflicting Software, indicating Something is:
◾ Interfering with libraries, processes, services or connectivity. This is not identified as a cheat, but is affecting the optimal ability of VAC to detect cheats reliably.
iv) Sub Optimal OS settings such as:
◾ Insecure boot configuration, i.e. Safe Mode, non-Secure Boot, Configuration Mode, or a MBR partition.
◾ Data Execution insecurities i.e. DEP disabled or Opted Out for Steam.exe
◾ Kernel level insecurities

Therefore there is no singular catch-all solution to every potential occurrence of this error, and each and every instance may be resultant from a variety of possible causes.
It is necessary, then, to ascertain the possible causes and eliminate their possible impact on a case by case basis.
Logic dictates the simplesta and fastest methods ought be attempted first given no prior information or knowlkedge of the user's system/settings.

SOLUTION:
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◾ Close any Steam games, Close Steam Client and then EXIT the Steam Client.
◾ Exit (not just close) all non-essential software. Disable such software from automatically staarting at bootup.
◾ Press R+WIN
◾ Enter the following and press return:

msconfig.exe

◾ Ensure 'Normal start' is selected.
◾ Click Apply then OK
◾ Press R+WIN
◾ Enter the following and press return:

%PROGRAMFILES(X86)%\Steam\bin\SteamService.exe" /repair

(Note that the path must reflect the actual location of the SteamService.exe file in the Steam installation directory.)
◾ Esure The currently logged on user has administrative privilege.
◾ Ensure the SteamApps directory and other Steam Games directories and the Steam install directory itself are accessible to SYSTEM, TRUSTEDINSTALLER, ADMINISTRATORS(Group) and the current USER
◾ Ensure Firewall is allowing all Steam and game traffic on the required ports under the correct protocol
◾ Ensure Steam, Game and related processes are exempt from anti-malware checking.
◾ Restart the computer.
◾ Open a command prompt or Powershell with administrative privilege
◾ In the command prompt, type the following commands and press Enter after each command:

bcdedit /deletevalue nointegritychecks
bcdedit /deletevalue loadoptions
bcdedit /debug off
bcdedit /deletevalue nx

If you receive the response "The value is protected by Secure Boot policy and cannot be modified or deleted" and have not altered the group security policy object, then all is well.
If you receive the response "The operation completed successfully" this implies that something had been set incorrectly and is now fixed. It may be worth checking this at later times to see if it remains as such, if not, possible malware is rendering your system compromised.
If you receive the response "An error occurred while attempting to delete the specified data element. The requested item could not be found" then this also means all is well, there are no optional parameters affecting the security.
◾ Now enter the following and press Return:

Dism.exe /Online /Cleanup-Image /RestoreHealth

(This may take a few minutes to complete.)
◾ When finsiehd, enter the following and press Return

sfc.exe /scannow

◾ Restart PC
◾ Test to see if issue persists.
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If the issue persists, proceed to the following part:
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◾ Refer to the guidance provided:

https://support.steampowered.com/kb_article.php?ref=2117-ilzv-2837

*◾ The following assumes:
◾ There are no 'cheats', have been no attempts to 'cheat' (in CSGO, TF2, or any other game) or otherwise modify/tamper with any files or processes in a manner which violates the SSA.
◾ Your computer is free of malware/viruses, is not part of a BotNet and has correctly configured firewall and antimalware software (exclusing the specific antimalware software mentioned in the Steam Support KB article)
◾ The advice provided by Valve's "Steam Support" knowledge management:

https://support.steampowered.com/kb_article.php?ref=2117-ilzv-2837

* has been followed fully, completely, without omission. Regardless of how importantly you regard your installation of "uTorrent", "TeamSpeak", "CCleaner", "IObit" software or "Cheat Engine" - none of these are essential to Steam and if you sincerely intend to fix the issue, then these should be removed at least until the issue is correctly resolved.
◾ You understand, acknowledge, accept and appreciate that the issue is due to settings, software or connectivity at your end.

______________________________________________________________________________________________________________
If the above has been followed without exception* AND THE ISSUE PERSISTS, please continue below.
______________________________________________________________________________________________________________
* Please ensure you have read every part very carefully and fulfilled EVERY STEP. This includes a complete restart of your computer between any insallation/uninstallation and COMPLETE REMOVAL OF ANY CONFLICTING SOFTWARE.

NOTE: For the following use the Stead AppID number (730 refers to CSGO), (for TF2 please substitute 440).
◾ Exit Steam
◾ WIN+R

steam://flushconfig
steam://uninstall/730
steam://checksysreqs/730

◾ Now open the installation directory for the product and remove all files.
◾ Restart Computer
◾ WINDOWS+R

steam://install/730

◾ Restart Computer
◾ WIN+R

steam://support/?Issues=*

◾ Check the Steam Console output (You may need to enable the console if not already) for any issues. If you encounter any, please post them. Assuuming there are none, please continue:
◾ Restart Computer
◾ Test the game.
______________________________________________________________________________________________________________
If the issue persists, please continue to the following part:
______________________________________________________________________________________________________________

Note: If your "computer" is a Laptop/Notebok/Netbook/Tablet or has been pre-built or purchased with Bundled software or installed with OEM OS,
Then software, OS customisation or policy changes may invalidate the ability to effectively remove all non-essential software, or may contain settings which override certain boot configurtation options etc. You may refer to your supplier.

◾ Please post a complete list of running processes and services.
◾ If any of the conflicting software referenced by the Steam Support article remain, then remove it.
______________________________________________________________________________________________________________
If the issue persists, a drastic, but guaranteed solution is as follows::
______________________________________________________________________________________________________________
◾ Remove ALL non-essential hardware (i.e. retain only GPU (if present), CPU, RAM, NIC (if present), Hard-Drive, Keyboard, Mouse, Monitor)
◾ Reformat HardDrive
◾ Install Windows (Restart)
◾ Install authentic and signed drivers (Restart)
◾ Enable and configure Windows Firewall
◾ Enable and configure Windows Defender
◾ Update Windows (Restart)
◾ Ensure Kernel Debugging is disabled and BCD is secure.
◾ Install Steam (Restart)
◾ Enable Hardware Firewall (if present) to allow Steam processes & services
◾ Enable Router port forwarding to the relevant port numbers, protocols and IP addresses for Steam, Steam Services and Games.
◾ Enable Windows Firewall to allow Steam processes & services
◾ Enable Windows Defender to allow Steam processes & services (Restart)
◾ Enable DEP for Steam processes and services.
◾ Update Steam (Restart)
◾ Install CSGO (Restart)
◾ Enable Windows Firewall to allow CSGO processes & services
◾ Enable Windows Defender to allow CSGO processes & services (Restart)
◾ Update CSGO (Restart)
◾ Run Steam
◾ Ensure connectivity, Steam Service and functionality.
◾ Run CSGO
◾ Ensure connectivity, CSGO funcitonality and settings
◾ Restart
◾ Disable/End non-essential processes and services.
◾ Start Steam
◾ Start CSGO
◾ Test operation and funcitonality in non-competitive modes to prevent unexpected cooldowns.
◾ End

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This fix will remain permanent through Windows, Steam and Game Updates, also thorugh Hardware Driver update Installtion.
However, installing 3rd party software, hardware, changing system settings or connectivity will render any such guarantee void.

[zachjd]

Good information. I hope it will help solve those issues some are experiencing.

[oguzton]

I fixed the error by doing: Verifying the game cache everytime after using ccleaner, Advanced system care and such programs. For me it was those programs that caused vac error.

[Commander Makara]

Zaraki Kenpachi a écrit :

I fixed the error by doing: Verifying the game cache evertime after using ccleaner, Advanced system care and such programs. For me it was those programs that caused vac error.

Yes, these programs are known to conflict.
They were mentionedin in the oiriginal post:

]
iii) Conflicting Software, indicating Something is:
◾ Interfering with libraries, processes, services or connectivity. This is not identified as a cheat, but is affecting the optimal ability of VAC to detect cheats reliably.

The advice provided by Valve's "Steam Support" knowledge management:
https://support.steampowered.com/kb_article.php?ref=2117-ilzv-2837
* has been followed fully, completely, without omission. Regardless of how importantly you regard your installation of "uTorrent", "TeamSpeak", "CCleaner", "IObit" software or "Cheat Engine" - none of these are essential to Steam and if you sincerely intend to fix the issue, then these should be removed at least until the issue is correctly resolved.

but also within Valve's knowledge article on the matter.

I have since compiled the above into a more accessibble 'Guide' so it is easire to find and read.

[Commander Makara]

COMPLETE GUIDE IS NOW HERE:

http://steamcommunity.com/sharedfiles/filedetails/?id=532288039

[oguzton]

Thanks for the Guide and all the information.
I think this post should be stickied. It is helpful for people complaining about the vac isaue.

[venom]

It is ok if we are kicked out in the middle of the game or deranked but the cooldown goes for 7 days .... I am a honest player and getting deranked from LEM TO NOVA because of this problem doesn't seem legit

[ruphus]

Another good post on VAC Discussion. :)

[Commander Makara]

killerwave a écrit :

It is ok if we are kicked out in the middle of the game or deranked but the cooldown goes for 7 days .... I am a honest player and getting deranked from LEM TO NOVA because of this problem doesn't seem legit

Any singular, first-time infraction resulting in a cooldown will only be for 30 minutes.
The only way to incur greater cooldown times is by contnuing to violate the conditions of play without allowing the 'cooldownb duration' to subside - usually this is 7 days per 'cooldown-duration-level'
I believe it's quite clear that abandoning games will result in such and that you are requested only to agree to continue provided you can guarantee you will play for the entire duration of the match.

HONESTY is keeping your word. Regardless of where the fault ultimately lies, you accepted full resposibility AND must explicitly agree to play the ENTIRE DURATION of the match.
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I actually returned to this thread to note a further change to the guide, that the secure boot checking necessitates a singular, secure, UEFI boot device. Dual-Booting or otherwise using legacy boot sector or BIOS etc. will entail less secure UEFI setting and therefore may fail the required levels of security for Windows and therefore VAC.