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- ----------------------------------------------------------------------
Title:      Server Response To SMTP Client EHLO Command Results In
            Buffer Overrun (Q326322)
Date:       24 July 2002
Software:   Microsoft Exchange 5.5
Impact:     Ability to run arbitrary code
Max Risk:   Medium
Bulletin:   MS02-037

Microsoft encourages customers to review the Security Bulletin at: 
http://www.microsoft.com/technet/security/bulletin/MS02-037.asp.
- ----------------------------------------------------------------------

Issue:
======
Technical description: 

The Internet Mail Connector (IMC) enables Microsoft Exchange Server
to communicate with other mail servers via SMTP. When the IMC
receives an SMTP extended Hello (EHLO) protocol command from a
connecting SMTP server, it responds by sending a status reply that
starts with the following:
250-<Exchange server ID>Hello<Connecting server ID> 

Where: 
<Exchange server ID> is the fully-qualified domain name (FQDN) of
the Exchange server <Connecting server ID> is either the FQDN or
the IP address of the server that initiated the connection.

The FQDN would be used if the Exchange5.5 IMC is able to resolve
this  information through a reverse DNS lookup; the IP address
would be used if a reverse DNS lookup was not possible or failed
to resolve the connecting servers IP address. 

A security vulnerability results because of an unchecked buffer
In the IMC code that generates the response to the EHLO protocol
command. If the total length of the message exceeds a particular
value, the data would overrun the buffer. If the buffer were
overrun with random data, it would result in the failure of the
IMC. If, however, the buffer were overrun with carefully chosen
data, it could be possible for the attacker to run code in the
security context of the IMC, which runs as Exchange5.5 Service
Account. 

It is important to note that the attacker could not simply send
Data to the IMC in order to overrun the buffer. Instead, the
Attacker would need to create a set of conditions that would
cause the IMC to overrun its own buffer when it generated the
EHLO response. Specifically, the attacker would need to ensure
that a reverse DNS lookup would not only succeed, but would
provide an FQDN whose length was sufficient to result in the
buffer overrun. 

Mitigating Factors:
==================== 

 - Creating an environment in which the IMC's reverse DNS lookup
   would not only succeed but also result in the buffer being 
   overrun would be difficult. The attacker could set up a rogue
   DNS server and manually populate the bogus FQDN information
   on it, but in this would require that the attacker have some
   means of forcing the IMC to consult the rogue DNS server when
   performing the reverse DNS lookup.

 - The IMC can be disabled for cases where SMTP support is not
   needed. If this has been done, the vulnerability could not be
   exploited. 

 - Customers can disable Reverse DNS lookup on EHLO by setting a
   registry key as defined in Q190026. The vulnerability could
   not be exploited on a system configured in such a way. 

 - If the buffer overrun caused the IMC to fail, normal service
   Could be restored by restarting the Exchange 5.5 IMC service.


Risk Rating:
============
 - Internet systems: Moderate
 - Intranet systems: Moderate
 - Client systems:   None

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms02-037.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - Microsoft thanks  Dan Ingevaldson of Internet Security Systems
   http://www.iss.net for reporting this issue to us and working
   with us to protect customers.
- ---------------------------------------------------------------------

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS 
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE 
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE
FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN
IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF
THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE
EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR 
INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

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