Injecting .NET Assembly to an Unmanaged Process

This is a quick lab to see what API sequence makes it possible to inject C# .NET assemblies / PE files (.exe and .dll) into an unmanaged process and invoke their methods.

This is the technique that makes execute-assembly command possible in Cobalt Strike.

Overview

At a high level, it works as follows:

CLRCreateInstance is used to retrieve an interface ICLRMetaHost
ICLRMetaHost->GetRuntime is used to retrieve ICLRRuntimeInfo interface for a specified CLR version
ICLRRuntimeInfo->GetInterface is used to load the CLR into the current process and retrieve an interface ICLRRuntimeHost
ICLRRuntimeHost->Start is used to initialize the CLR into the current process
ICLRRuntimeHost->ExecuteInDefaultAppDomain is used to load the C# .NET assembly and call a particular method with an optionally provided argument

Code

unmanaged.cpp (in my lab compiled to LoadCLR.exe) - a C++ program that loads a C# assembly
CLRHello1.exe and invokes its method spotlessMethod
managed.cs (in my lab compiled to CLRHello1.exe) - a C# program that is loaded by the unmanaged process (LoadCLR.exe). It has a method spotlessMethod that is invoked via ExecuteInDefaultAppDomain.O

Once invoked, the spotlessMethod prints out Hi from CLR to the console window.

// code mostly stolen from pabloko's comment in https://gist.github.com/xpn/e95a62c6afcf06ede52568fcd8187cc2
#include <iostream>
#include <metahost.h>
#include <corerror.h>
#pragma comment(lib, "mscoree.lib")

int main()
{
    ICLRMetaHost* metaHost = NULL;
    ICLRRuntimeInfo* runtimeInfo = NULL;
    ICLRRuntimeHost* runtimeHost = NULL;
    DWORD pReturnValue;

    CLRCreateInstance(CLSID_CLRMetaHost, IID_ICLRMetaHost, (LPVOID*)&metaHost);
    metaHost->GetRuntime(L"v4.0.30319", IID_ICLRRuntimeInfo, (LPVOID*)&runtimeInfo);
    runtimeInfo->GetInterface(CLSID_CLRRuntimeHost, IID_ICLRRuntimeHost, (LPVOID*)&runtimeHost);
    runtimeHost->Start();
    HRESULT res = runtimeHost->ExecuteInDefaultAppDomain(L"C:\\labs\\CLRHello1\\CLRHello1\\CLRHello1\\bin\\Debug\\CLRHello1.exe", L"CLRHello1.Program", L"spotlessMethod", L"test", &pReturnValue);
    if (res == S_OK)
    {
        std::cout << "CLR executed successfully\n";
    }
    
    runtimeInfo->Release();
    metaHost->Release();
    runtimeHost->Release();
    return 0;
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CLRHello1
{
    class Program
    {
        static void Main(string[] args)
        {
            return;   
        }
        
        // important: methods called by ExecuteInDefaultAppDomain need to stick to this signature
        static int spotlessMethod(String pwzArgument)
        {
            Console.WriteLine("Hi from CLR");
            return 1;
        }
    }
}

Demo

Below shows how LoadCLR.exe loaded our C# assembly CLRHello.exe (seen in LoadCLR.exe loaded modules tab) and invoked the spotlessMethod, that printed Hi from CLR to the console: