VAC Authentication Error Reliable Solution

The "VAC authentication error" or "A problem with your computer is blocking the VAC system" is somewhat 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.

This Guide provides a definitive, complete solution not only for any occurrence of the issue, but also (and perhaps more crucially) how to prevent its reoccurrence in future.

The solution presented is guaranteed to work as described.

What Is The VAC Authentication Error?

UPDATE: In more recent times, the displayed error massage has been amended to describe the potential reason for the error, not in detail, but at least as an overview to provide a little more context.

UPDATE: A number of reports claim incorrectly that CCleaner cannot result in this issue. Typically the default settings for CCleaner are unlikely to have any noticeable impact on Steam. However, features such as the realtime monitoring availability, DNS Cache flushing, and CCEnhancer as well as certain options pertianing to Valve's Steam services have been specifically identified as resulting in the VAC authentication issue by our dedicated testing.

The "VAC authentication error" or "A problem with your computer is blocking the VAC system" is somewhat 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.

VAC is Valve's Anti-Cheat system and to operate effectively, it needs access to files and to monitor certain memory areas (specifically for cheats and cheat-like processes which may accompany certain security problems) and to be able to transmit the status back to Valve. This is all according to permissions and the agreement you made with Valve.
VAC does not scrutinise your personal file contents, transmit details of your internet browsing habits, nor does it even affect any files or data that are quite simply, none of Valve's business whatsoever.

As cheats have grown more complex and the attempts to hide them have grown increasingly more technical, VAC has therefore needed to be a little more stringent and thorough in its endeavour. Combined with astronomically immense number of possible degrees of freedom available in consideration of software installation, user settings and operating system modification, the potential for a seemingly inoccuous installation or change can have a definite impact on the optimal performance of VAC.

Since VAC is designed to apply permanent bans with no appeal, it is important that only valid detections are counted. Every possibility, no matter how slight, must be correctly mitigated. The public uproar/backlash for a false ban going unaddressed would be horrendous (Even when an accidental human erorr resulted in thousands of banned accounts (which of courtse were reversed within a couple of days), various media exploded in frenzy). Therefore, it is understandably critical that VAC is allowed to function properly without hindrance.

This error, is what occurs if it is prevented from doing so.

Why am I receiving this error?

First, understand that this is not a VAC ban.
There is absolutely no accusation of cheating.
The appearance of this error message is as simple (if vague) as the message itself describes:
"A problem with your computer is blocking VAC"

The discrete potential reasons for this occurrence are too numerous to name, but may be classified within 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 policy
◾ The file is missing or corrupted.
ii) Connectivity issues. These may be:
◾ Bad / intermittent conenction.
◾ Incorrectly configured Firewall, disabled / 'emergency mode' Firewall setting
◾ Anti-Malware / "Safe Mode" 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.
◾ Further to the above, 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.
◾ Data Execution insecurities i.e. DEP disabled or Opted Out for Steam.exe
◾ Kernel level insecurities
◾ Virtualisation or 'sandboxing' isolating required runtime components from VAC's access.
◾ Certain OEM modifications and bundled software may override firmware and/or result in modified EFI etc. which again, will be considered by Windows as less-secure, and therefore, VAC will do likewise.

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.

• "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 might say 'bloated') delicate, precise instruments. 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 type of activity generates and alters many areas from log files, caches, registry entriess, temporary files, NT data stream access records/USN jounral entry, prefetch records, search indices, MRU listings 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 at all for those cheat developers and public demand for a solution, there would be no VAC and no unnecessary activity from Valve.
• "My computer is brand new; nothing could be causing this!"
Clearly, you have modified some aspects of the computer, since you must have, at the very least, installed Steam and the game in which this error appears.These two features themselves will not cause the error, but it highlights the error in your judgement. As mentioned earlier, even a seemingly inoccous, or unrelated software installation/settings change may have considerable impact.
• "But - software surely wont affect VAC, will it? It's not a cheat"
It's not a cheat, and had it been possible to cheat with said software, then the account would be banned, not given an error message. However, some very popular software is well known to cause this error due to how it interacts with the Operating system:

CCleaner
iObit software
"Start Menu" software for Windows 8, 8.1 and 10
McAfee security software

To name but a few.

The issue here is not that the software is involved with cheating per se, but in its ability to prevent VAC from opeating effectively.

Process for Solution

Given that there are so many possible causes for this error to occur specifically per user and device, it is important to maintain a methodical and organised process to reach a solution.

Logically it seems most prudent to begin with the most basic, straightforward and hopefully efficient points in priority, with the long-winded, more drastic solution as a 'last resort'.

This section of the guide simply outlines the overall sequence of action the guide will pursue in finding the appropriate solution for the user.

• Determine status
• Remove possible conflicts
• Ensure Software is up to date
• Ensure adequate permittivity per security policy
• Follow Steam Support knowledge management advice
• Re-Install Steam and game Software
• Coarse solution when the above is unsuccessful

Solution (Software System)

Software System

This first step to solving the problem ensures that Windows system and Steam settings are functional and not critically corrupted.

• Ensure Windows is up to date
• Fully EXIT Steam and any Steam games
• Run full, complete antimalware scan and fully remove any traces of malware discovered.
• Restart the computer.
• Press WINDOWS + R
• Enter the following: "%PROGRAMFILES(X86)%\Steam\bin\SteamService.exe" /repair
(Please note that the location of the Steam Installation may be different. If so, amend as required)
• Esure The currently logged on user has administrative level security privileges.
• 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
• Check for Steam Client Updates and install if necessary
• Press WINDOWS + R
• Enter the following:
  steam://flushconfig
• Press WINDOWS + R
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 starting at bootup.
• Press WINDOWS + R
• Enter the following:
  msconfig.exe
• On first (General) tab, Ensure 'Normal start' is selected.
• On the Boot tab, Ensure 'Safe boot' is unselected.
• Click Apply then OK
• Restart the computer.
• Enter UEFI setting
• Enable only secure boot UE firmware interface. Do not allow legacy BIOS or Dual Boot setups.
• Save UEFI Settings
• Restart 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 in the command prompt and press Return:
  Dism.exe /Online /Cleanup-Image /RestoreHealth
(This may take a few minutes to complete)

• When finsihed, enter the following in the command prompt and press Return:
  sfc.exe /scannow
• Restart the computer.

Solution (Permittivity & Connectivity)

Permittivity & Connectivity

This section details allowing connectivity and permittivity for Steam functionality. It assumes that all steps from the previous solution topic have been completed correctly.

• Check with your ISP that your service is reliable and consistent.
• Reset Modem/Router
• Restart Computer
• Ensure The currently logged on user has administrative level security privileges.
• 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
• Exit (not just close) all non-essential software. Disable such software from automatically starting at bootup.
• Ensure any Router device is set to forward requests to the relevant Steam and/or game ports for all protocols
• Disable all but ONE anti-malware solution and ONE firewall solution. Recommended (for Windows) are Windows Defender and Windows Firewall. Multiple solutions concurrently will cause problems.
• Prevent any disabled processes from starting at Bootup. Fully uninstall if necessary.
• Ensure msconfig.exe is operating correctly. If not, run DISM.exe to restore health from the image and then run SFC.exe with the scannow parameter.
• Now enter the following in the command prompt and press Return:
  
• Dism.exe /Online /Cleanup-Image /RestoreHealth
• (This may take a few minutes to complete)
  
• When finsihed, enter the following in the command prompt and press Return:
  
• sfc.exe /scannow
• Restart the computer.
• Ensure solutions for Antimalware and Antivirus are not marked as possible conflicting software on Steam Support knowledge article
• Ensure enabled solution is Enabled and effective.
• Ensure Firewall is correctly configured to allow all Steam and Steam game traffic on necessary ports and protocols.
• Fully exclude Steam, associate processes (i.e SteamWebHelper.exe) and Service (SteamService.exe) completely from antimalware scanning.
(Please note that many Antimalware solutions offer 'Realtime' protection features which will examine any calls to running processes and/or their dependencies. Steam and the game features must be excluded from these features.)
• Press WINDOWS + R
• Enter the following: "%PROGRAMFILES(X86)%\Steam\bin\SteamService.exe" /repair
(Please note that the location of the Steam Installation may be different. If so, amend as required)
• Add the following to the Steam shortcut CL parameter argument list:
  -clearbeta
• Restart Computer

Solution (Steam Support Knowledge Management)

Steam Support Knowledge Management

Steam Support provide an article with details solutions for the majority of cases It is imperative that all the steps are carried out. Experience has shown that the majority of those experiencing problems claiming the article did not provide a solution have since been found to have not completed all the necessary steps.
Typically, not running Command Prompt with administrative privilege or failing to fully remove known conflicting software are the most common reasons for failure.

Once again, this solution assumes that the previous two solution documents have been completed in full.

• Open a command prompt or Powershell with administrative level security privileges
• 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.
• Refer to the guidance provided: https://support.steampowered.com/kb_article.php?ref=2117-ilzv-2837
*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.
• Press WINDOWS + R
• Enter the following, press ENTER after each line:
  steam://uninstall/730
  steam://flushconfig
  steam://checksysreqs/730
(Please Note that for the above use the Steam AppID number (730 refers to CSGO), (for TF2 please substitute 440))
• Open the installation directory for the product and remove all files
It is suggested that Saved Games are backed up if necessary.
• Restart Computer
• Press WINDOWS + R
• Enter the following:
  steam://install/730
(Please Note that for the above use the Steam AppID number (730 refers to CSGO), (for TF2 please substitute 440))
• Restart Computer
• Press WINDOWS + R
• Enter the following, press ENTER after each line:
  steam://support/?Issues=*
• Check the Steam Console output (You may need to enable the console if not already) for any issues.
  
• Restart Computer

Solution (Course Final Resort Solution)

Course Final Resort

This is a last-ditch solution for when all previous attempts have been unsuccessful. It is not receommended this solution be used until all other options are exhausted since it involves a lot of time and other solutions may be easier, quicker and more worthwhile.

However, this is guaranteed to work absolutely, completely and permanently in all situations.*

• Shutdown computer (Not Sleep/Hibernate)
• 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 and OS Boots into Normal (not Debugging/Safe) Mode
• 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 Steam's relevant subscriptions
• 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 relevant subscriptions via Steam (Restart)
• Enable Windows Firewall to allow relevant subscriptions via Steam processes & services
• Enable Windows Defender to allow relevant subscriptions via Steam processes & services (Restart)
• Update relevant subscriptions via Steam (Restart)
• Run Steam
• Ensure connectivity, Steam Service and functionality
• Run relevant subscriptions via Steam
• Ensure connectivity, relevant subscriptions' functionality and settings
• Restart
• Disable/End ALL non-essential processes and services
• Start Steam
• Start relevant subscriptions via Steam
• Test operation and functionality in non-competitive modes to prevent unexpected cooldowns
* Situations exempt are OEM bundled machines (and equivalently, certain tablets, phones, laptop/notebook/netbook type computers) with modified OS or Dual-Boot setups with legacy BIOS boot options, Any reinstallation of conflicting software/hardware or modification of conflicting settings, later corruption.However, any Windows, Steam Client, Game and essential, official Driver installation will not affect the status of this solution.

[dustStrikeR]

it didn't worked for me anyway, i fixed it already, all steps correctly, not a single error trough the process

[dustStrikeR]

* Situations exempt are OEM bundled machines (and equivalently, certain tablets, phones, laptop/notebook/netbook type computers) with modified OS or Dual-Boot setups with legacy BIOS boot options, Any reinstallation of conflicting software/hardware or modification of conflicting settings, later corruption.However, any Windows, Steam Client, Game and essential, official Driver installation will not affect the status of this solution.

[Commander Makara]

This guide does not support laptop devices as many laptop providers customise in the same manner as bundled Desktop builds which can result in non-standard configurations and policy settings changed from default)

In the "Permittivity and Connectivity" section of the guide it is #4 that states:

Ensure The currently logged on user has administrative level security privileges.
Had you followed this correctly, this would have resolved your issue.

[dustStrikeR]

it is a laptop, which i'm using since 2012, been using steam for years now. to this moment i already fixed the problem with a step that didn't saw on your guide. i simply changed steam properties to always run with admin privileges, with that, windows is no longer blocking vac communication. to be honest i still don't know how that problem started, even with all the changes proposed on your guide vac wouldn't communicate because of admin privileges that will give steam the necessary permissions.

[Commander Makara]

are you using desktop or laptop?
is the computer prebuild?

[dustStrikeR]

didn't work for me, none of those options

[Chris.Mp7]

Damn man unfortunatly i got this on tf2.This is all too complicated for me so.I guess im done :(

[Commander Makara]

T-Bone, are you still experiencing issues?

[Stanley Goodspeed]

this is all way to advanced for me... i guess im screwed

[Commander Makara]

AzKat, thanks for that snippet of information.

Resetting the router / MoDeM is listed as the second step in ""Permittivity & Connectivity"" section of the guide.