MS03-046 Microsoft Exchange 2000 Heap Overflow

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