How to create a native C++ DLL that I can run with rundll32.exe without specifying an entry point

Question

How I will be able to create a native dll that I can execute with rundll32.exe without specifying an entry point:

Example:

C: \> rundll32.exe mydll.dll

I created a DLL project on visual studio but I don't know where to put my code:

• DLL project template generated by Visual Studio

// dllmain.cpp : Définit le point d'entrée de l'application DLL.
#include "pch.h"

BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

• my code that I want to run with the dll:

#include <stdio.h>
#include <stdlib.h>

// enable cross compiling
#ifdef __linux__
#include <sys/mman.h>
#elif _WIN32 || _MINGW_
#include <windows.h>
#endif


void main()
{
    const char shellcode[] = "\x48\x83\xec\x48\x48\x83\xe4\xf0\x4c\x8d\x44\x24\x28\x48\x8d\x15\xda\x01\x00\x00\x48\x8d\x0d\xc6\x01\x00\x00\xe8\x64\x00\x00\x00\x4c\x8b\x4c\x24\x28\x4c\x8d\x44\x24\x30\x48\x8d\x15\xca\x01\x00\x00\x48\x8d\x0d\xa9\x01\x00\x00\xe8\x47\x00\x00\x00\x48\x8d\x0d\xc3\x01\x00\x00\xff\x54\x24\x28\x4c\x8b\x4c\x24\x28\x4c\x8d\x44\x24\x38\x48\x8d\x15\xb9\x01\x00\x00\x48\x8d\x0d\xa7\x01\x00\x00\xe8\x1f\x00\x00\x00\x4d\x31\xc9\x4c\x8d\x05\xcb\x01\x00\x00\x48\x8d\x15\xa8\x01\x00\x00\x48\x31\xc9\xff\x54\x24\x38\x48\x31\xc9\xff\x54\x24\x30\x48\x81\xec\x68\x01\x00\x00\x48\x89\x5c\x24\x28\x48\x89\x6c\x24\x30\x48\x89\x7c\x24\x38\x48\x89\x74\x24\x40\x4c\x89\x64\x24\x48\x4c\x89\x6c\x24\x50\x4c\x89\x74\x24\x58\x4c\x89\x7c\x24\x60\x65\x4c\x8b\x1c\x25\x60\x00\x00\x00\x4d\x8b\x5b\x18\x4d\x8d\x5b\x10\x4d\x89\xdf\x4d\x8b\x1b\xfc\x49\x8b\x7b\x60\x48\x89\xce\xac\x84\xc0\x74\x26\x8a\x27\x80\xfc\x61\x7c\x03\x80\xec\x20\x38\xc4\x75\x08\x48\xff\xc7\x48\xff\xc7\xeb\xe5\x4d\x8b\x1b\x4d\x39\xfb\x75\xd6\x48\x31\xc0\xe9\xb1\x00\x00\x00\x49\x8b\x5b\x30\x44\x8b\x63\x3c\x49\x01\xdc\x49\x81\xc4\x88\x00\x00\x00\x45\x8b\x2c\x24\x4d\x85\xed\x75\x08\x48\x31\xc0\xe9\x8e\x00\x00\x00\x4e\x8d\x1c\x2b\x45\x8b\x74\x24\x04\x4d\x01\xee\x41\x8b\x4b\x18\x45\x8b\x53\x20\x49\x01\xda\xff\xc9\x4d\x8d\x24\x8a\x41\x8b\x3c\x24\x48\x01\xdf\x48\x89\xd6\xa6\x75\x08\x8a\x06\x84\xc0\x74\x09\xeb\xf5\xe2\xe5\x48\x31\xc0\xeb\x55\x45\x8b\x63\x24\x49\x01\xdc\x66\x41\x8b\x0c\x4c\x45\x8b\x63\x1c\x49\x01\xdc\x41\x8b\x04\x8c\x4c\x39\xe8\x7c\x36\x4c\x39\xf0\x73\x31\x48\x8d\x34\x18\x48\x8d\x7c\x24\x68\xa4\x80\x3e\x2e\x75\xfa\xa4\xc7\x07\x44\x4c\x4c\x00\x4d\x89\xc6\x48\x8d\x4c\x24\x68\x41\xff\xd1\x4d\x89\xf0\x48\x8d\x4c\x24\x68\x48\x89\xf2\xe9\x08\xff\xff\xff\x48\x01\xd8\x49\x89\x00\x48\x8b\x5c\x24\x28\x48\x8b\x6c\x24\x30\x48\x8b\x7c\x24\x38\x48\x8b\x74\x24\x40\x4c\x8b\x64\x24\x48\x4c\x8b\x6c\x24\x50\x4c\x8b\x74\x24\x58\x4c\x8b\x7c\x24\x60\x48\x81\xc4\x68\x01\x00\x00\xc3\x4b\x45\x52\x4e\x45\x4c\x33\x32\x2e\x44\x4c\x4c\x00\x4c\x6f\x61\x64\x4c\x69\x62\x72\x61\x72\x79\x41\x00\x45\x78\x69\x74\x50\x72\x6f\x63\x65\x73\x73\x00\x55\x53\x45\x52\x33\x32\x2e\x44\x4c\x4c\x00\x4d\x65\x73\x73\x61\x67\x65\x42\x6f\x78\x41\x00\x48\x69\x20\x66\x72\x6f\x6d\x20\x69\x6e\x6a\x65\x63\x74\x65\x64\x20\x73\x68\x65\x6c\x6c\x63\x6f\x64\x65\x21\x00\x53\x68\x65\x6c\x6c\x63\x6f\x64\x65\x54\x6f\x4a\x53\x63\x72\x69\x70\x74\x20\x50\x6f\x43\x20\x00";
    PVOID shellcode_exec = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
    RtlCopyMemory(shellcode_exec, shellcode, sizeof shellcode);
    DWORD threadID;
    HANDLE hThread = CreateThread(NULL, 0, (PTHREAD_START_ROUTINE)shellcode_exec, NULL, 0, &threadID);
    WaitForSingleObject(hThread, INFINITE);
}

Answer 1

The entry point parameter of rundll32 is not optional , you MUST specify an entry point, otherwise rundll32 won't know which function to call. There is no "default" entry point.

To be callable by rundll32, you must export a function with one of the following signatures:

void CALLBACK EntryPoint(HWND hwnd, HINSTANCE hinst, LPSTR pszCmdLine, int nCmdShow)

void CALLBACK EntryPointA(HWND hwnd, HINSTANCE hinst, LPSTR pszCmdLine, int nCmdShow)

void CALLBACK EntryPointW(HWND hwnd, HINSTANCE hinst, LPWSTR pszCmdLine, int nCmdShow)

For example:

#include "pch.h"

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>

BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

__declspec(dllexport) void CALLBACK myFunc(HWND, HINSTANCE, LPSTR, int)
{
    const char shellcode[] = "\x48\x83\xec\x48\x48\x83\xe4\xf0\x4c\x8d\x44\x24\x28\x48\x8d\x15\xda\x01\x00\x00\x48\x8d\x0d\xc6\x01\x00\x00\xe8\x64\x00\x00\x00\x4c\x8b\x4c\x24\x28\x4c\x8d\x44\x24\x30\x48\x8d\x15\xca\x01\x00\x00\x48\x8d\x0d\xa9\x01\x00\x00\xe8\x47\x00\x00\x00\x48\x8d\x0d\xc3\x01\x00\x00\xff\x54\x24\x28\x4c\x8b\x4c\x24\x28\x4c\x8d\x44\x24\x38\x48\x8d\x15\xb9\x01\x00\x00\x48\x8d\x0d\xa7\x01\x00\x00\xe8\x1f\x00\x00\x00\x4d\x31\xc9\x4c\x8d\x05\xcb\x01\x00\x00\x48\x8d\x15\xa8\x01\x00\x00\x48\x31\xc9\xff\x54\x24\x38\x48\x31\xc9\xff\x54\x24\x30\x48\x81\xec\x68\x01\x00\x00\x48\x89\x5c\x24\x28\x48\x89\x6c\x24\x30\x48\x89\x7c\x24\x38\x48\x89\x74\x24\x40\x4c\x89\x64\x24\x48\x4c\x89\x6c\x24\x50\x4c\x89\x74\x24\x58\x4c\x89\x7c\x24\x60\x65\x4c\x8b\x1c\x25\x60\x00\x00\x00\x4d\x8b\x5b\x18\x4d\x8d\x5b\x10\x4d\x89\xdf\x4d\x8b\x1b\xfc\x49\x8b\x7b\x60\x48\x89\xce\xac\x84\xc0\x74\x26\x8a\x27\x80\xfc\x61\x7c\x03\x80\xec\x20\x38\xc4\x75\x08\x48\xff\xc7\x48\xff\xc7\xeb\xe5\x4d\x8b\x1b\x4d\x39\xfb\x75\xd6\x48\x31\xc0\xe9\xb1\x00\x00\x00\x49\x8b\x5b\x30\x44\x8b\x63\x3c\x49\x01\xdc\x49\x81\xc4\x88\x00\x00\x00\x45\x8b\x2c\x24\x4d\x85\xed\x75\x08\x48\x31\xc0\xe9\x8e\x00\x00\x00\x4e\x8d\x1c\x2b\x45\x8b\x74\x24\x04\x4d\x01\xee\x41\x8b\x4b\x18\x45\x8b\x53\x20\x49\x01\xda\xff\xc9\x4d\x8d\x24\x8a\x41\x8b\x3c\x24\x48\x01\xdf\x48\x89\xd6\xa6\x75\x08\x8a\x06\x84\xc0\x74\x09\xeb\xf5\xe2\xe5\x48\x31\xc0\xeb\x55\x45\x8b\x63\x24\x49\x01\xdc\x66\x41\x8b\x0c\x4c\x45\x8b\x63\x1c\x49\x01\xdc\x41\x8b\x04\x8c\x4c\x39\xe8\x7c\x36\x4c\x39\xf0\x73\x31\x48\x8d\x34\x18\x48\x8d\x7c\x24\x68\xa4\x80\x3e\x2e\x75\xfa\xa4\xc7\x07\x44\x4c\x4c\x00\x4d\x89\xc6\x48\x8d\x4c\x24\x68\x41\xff\xd1\x4d\x89\xf0\x48\x8d\x4c\x24\x68\x48\x89\xf2\xe9\x08\xff\xff\xff\x48\x01\xd8\x49\x89\x00\x48\x8b\x5c\x24\x28\x48\x8b\x6c\x24\x30\x48\x8b\x7c\x24\x38\x48\x8b\x74\x24\x40\x4c\x8b\x64\x24\x48\x4c\x8b\x6c\x24\x50\x4c\x8b\x74\x24\x58\x4c\x8b\x7c\x24\x60\x48\x81\xc4\x68\x01\x00\x00\xc3\x4b\x45\x52\x4e\x45\x4c\x33\x32\x2e\x44\x4c\x4c\x00\x4c\x6f\x61\x64\x4c\x69\x62\x72\x61\x72\x79\x41\x00\x45\x78\x69\x74\x50\x72\x6f\x63\x65\x73\x73\x00\x55\x53\x45\x52\x33\x32\x2e\x44\x4c\x4c\x00\x4d\x65\x73\x73\x61\x67\x65\x42\x6f\x78\x41\x00\x48\x69\x20\x66\x72\x6f\x6d\x20\x69\x6e\x6a\x65\x63\x74\x65\x64\x20\x73\x68\x65\x6c\x6c\x63\x6f\x64\x65\x21\x00\x53\x68\x65\x6c\x6c\x63\x6f\x64\x65\x54\x6f\x4a\x53\x63\x72\x69\x70\x74\x20\x50\x6f\x43\x20\x00";
    PVOID shellcode_exec = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
    if (shellcode_exec) {
        RtlCopyMemory(shellcode_exec, shellcode, sizeof shellcode);
        DWORD dwOldProtect;
        if (VirtualProtect(shellcode_exec, sizeof shellcode, PAGE_EXECUTE, &dwOldProtect)) {
            FlushInstructionCache(GetCurrentProcess(), shellcode_exec, sizeof shellcode);
            DWORD threadID;
            HANDLE hThread = CreateThread(NULL, 0, (PTHREAD_START_ROUTINE)shellcode_exec, NULL, 0, &threadID);
            if (hThread) {
                WaitForSingleObject(hThread, INFINITE);
                CloseHandle(hThread);
            }
        }
        VirtualFree(shellcode_exec, 0, MEM_RELEASE);
    }
}

C: \> rundll32.exe mydll.dll,myFunc <optional parameters here>

And just FYI, you don't actually need the worker thread at all, you can just execute the shell code directly like any other function:

__declspec(dllexport) void CALLBACK myFunc(HWND hwnd, HINSTANCE hinst, LPSTR pszCmdLine, int nCmdShow)
{
    const char shellcode[] = "\x48\x83\xec\x48\x48\x83\xe4\xf0\x4c\x8d\x44\x24\x28\x48\x8d\x15\xda\x01\x00\x00\x48\x8d\x0d\xc6\x01\x00\x00\xe8\x64\x00\x00\x00\x4c\x8b\x4c\x24\x28\x4c\x8d\x44\x24\x30\x48\x8d\x15\xca\x01\x00\x00\x48\x8d\x0d\xa9\x01\x00\x00\xe8\x47\x00\x00\x00\x48\x8d\x0d\xc3\x01\x00\x00\xff\x54\x24\x28\x4c\x8b\x4c\x24\x28\x4c\x8d\x44\x24\x38\x48\x8d\x15\xb9\x01\x00\x00\x48\x8d\x0d\xa7\x01\x00\x00\xe8\x1f\x00\x00\x00\x4d\x31\xc9\x4c\x8d\x05\xcb\x01\x00\x00\x48\x8d\x15\xa8\x01\x00\x00\x48\x31\xc9\xff\x54\x24\x38\x48\x31\xc9\xff\x54\x24\x30\x48\x81\xec\x68\x01\x00\x00\x48\x89\x5c\x24\x28\x48\x89\x6c\x24\x30\x48\x89\x7c\x24\x38\x48\x89\x74\x24\x40\x4c\x89\x64\x24\x48\x4c\x89\x6c\x24\x50\x4c\x89\x74\x24\x58\x4c\x89\x7c\x24\x60\x65\x4c\x8b\x1c\x25\x60\x00\x00\x00\x4d\x8b\x5b\x18\x4d\x8d\x5b\x10\x4d\x89\xdf\x4d\x8b\x1b\xfc\x49\x8b\x7b\x60\x48\x89\xce\xac\x84\xc0\x74\x26\x8a\x27\x80\xfc\x61\x7c\x03\x80\xec\x20\x38\xc4\x75\x08\x48\xff\xc7\x48\xff\xc7\xeb\xe5\x4d\x8b\x1b\x4d\x39\xfb\x75\xd6\x48\x31\xc0\xe9\xb1\x00\x00\x00\x49\x8b\x5b\x30\x44\x8b\x63\x3c\x49\x01\xdc\x49\x81\xc4\x88\x00\x00\x00\x45\x8b\x2c\x24\x4d\x85\xed\x75\x08\x48\x31\xc0\xe9\x8e\x00\x00\x00\x4e\x8d\x1c\x2b\x45\x8b\x74\x24\x04\x4d\x01\xee\x41\x8b\x4b\x18\x45\x8b\x53\x20\x49\x01\xda\xff\xc9\x4d\x8d\x24\x8a\x41\x8b\x3c\x24\x48\x01\xdf\x48\x89\xd6\xa6\x75\x08\x8a\x06\x84\xc0\x74\x09\xeb\xf5\xe2\xe5\x48\x31\xc0\xeb\x55\x45\x8b\x63\x24\x49\x01\xdc\x66\x41\x8b\x0c\x4c\x45\x8b\x63\x1c\x49\x01\xdc\x41\x8b\x04\x8c\x4c\x39\xe8\x7c\x36\x4c\x39\xf0\x73\x31\x48\x8d\x34\x18\x48\x8d\x7c\x24\x68\xa4\x80\x3e\x2e\x75\xfa\xa4\xc7\x07\x44\x4c\x4c\x00\x4d\x89\xc6\x48\x8d\x4c\x24\x68\x41\xff\xd1\x4d\x89\xf0\x48\x8d\x4c\x24\x68\x48\x89\xf2\xe9\x08\xff\xff\xff\x48\x01\xd8\x49\x89\x00\x48\x8b\x5c\x24\x28\x48\x8b\x6c\x24\x30\x48\x8b\x7c\x24\x38\x48\x8b\x74\x24\x40\x4c\x8b\x64\x24\x48\x4c\x8b\x6c\x24\x50\x4c\x8b\x74\x24\x58\x4c\x8b\x7c\x24\x60\x48\x81\xc4\x68\x01\x00\x00\xc3\x4b\x45\x52\x4e\x45\x4c\x33\x32\x2e\x44\x4c\x4c\x00\x4c\x6f\x61\x64\x4c\x69\x62\x72\x61\x72\x79\x41\x00\x45\x78\x69\x74\x50\x72\x6f\x63\x65\x73\x73\x00\x55\x53\x45\x52\x33\x32\x2e\x44\x4c\x4c\x00\x4d\x65\x73\x73\x61\x67\x65\x42\x6f\x78\x41\x00\x48\x69\x20\x66\x72\x6f\x6d\x20\x69\x6e\x6a\x65\x63\x74\x65\x64\x20\x73\x68\x65\x6c\x6c\x63\x6f\x64\x65\x21\x00\x53\x68\x65\x6c\x6c\x63\x6f\x64\x65\x54\x6f\x4a\x53\x63\x72\x69\x70\x74\x20\x50\x6f\x43\x20\x00";
    PVOID shellcode_exec = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
    if (shellcode_exec) {
        RtlCopyMemory(shellcode_exec, shellcode, sizeof shellcode);
        DWORD dwOldProtect;
        if (VirtualProtect(shellcode_exec, sizeof shellcode, PAGE_EXECUTE, &dwOldProtect)) {
            PTHREAD_START_ROUTINE proc = (PTHREAD_START_ROUTINE) shellcode_exec;
            proc(NULL);
        }
        VirtualFree(shellcode_exec, 0, MEM_RELEASE);
    }
}