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Title:      Unchecked Buffer in Gopher Protocol Handler Can Run Code
            of Attacker's Choice (Q323889)
Date:       11 June 2002
Software:   Internet Explorer, Proxy Server, Internet Security and
            Acceleration Server
Impact:     Run Code of Attacker's Choice
Max Risk:   Critical
Bulletin:   MS02-027

Microsoft encourages customers to review the Security Bulletin at: 
http://www.microsoft.com/technet/security/bulletin/MS02-027.asp.
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Issue:
======
This is a work-around bulletin that details steps customers can
take to protect themselves against a publicly disclosed
vulnerability until patches are available. 

The Gopher protocol is a legacy protocol that provides for the
transfer of text-based information across the Internet.
Information on Gopher servers is hierarchically presented using a
menu system, and multiple Gopher servers can be linked together to
form a collective "Gopherspace". 

There is an unchecked buffer in a piece of code which handles the
response from Gopher servers. This code is used independently in
IE, ISA, and Proxy Server. A security vulnerability results
because it is possible for an attacker to attempt to exploit this
flaw by mounting a buffer overrun attack through a specially
crafted server response. The attacker could seek to exploit the
vulnerability by crafting a web page that contacted a server
under the attacker's control. The attacker could then either post
this page on a web site or send it as an HTML email. When the page
was displayed and the server's response received and processed,
the attack would be carried out. 

A successful attack requires that the attacker be able to send
information to the intended target using the Gopher protocol.
Anything which inhibited Gopher connectivity could protect against
attempts to exploit this vulnerability. In the case of IE, the
code would be run in the user's context. As a result, any
limitations on the user would apply to the attacker's code
as well. 


Mitigating Factors:
====================
 - A successful attack requires that the attacker's server be
   able to deliver information to the target using the Gopher
   protocol. Customers who block Gopher at the perimeter would be
   protected against attempts to exploit this vulnerability across
   the Internet. 

 - In the case of IE, code would run in the security context of
   the user. As a result, any limitations on the user's ability
   would also restrict the actions an attacker's code could take. 

 - A successful attack against ISA and Proxy servers would
   require that the malicious response be received by the web
   proxy service. In practical terms, this means that a proxy
   client would have to submit the initial request through the
   proxy server.

Risk Rating:
============
 - Internet systems: Critical
 - Intranet systems: Critical
 - Client systems: Critical

Patch Availability:
===================
 - A patch is currently under development to fix this
   vulnerability. Please read the Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms02-027.asp
   for workaround information while patches are developed.

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