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Title:      ISA Server H.323 Gatekeeper Service Contains Memory Leak
Date:       16 August 2001
Software:   ISA Server 2000
Impact:     Denial of service, cross-site scripting
Bulletin:   MS01-045

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-045.asp.
- ----------------------------------------------------------------------

Issue:
======
This bulletin discusses three security vulnerabilities that are
unrelated except in the sense that both affect ISA Server 2000: 

 - A denial of service vulnerability involving the H.323 Gatekeeper
   Service, a service that supports the transmission of voice-over-IP
   traffic through the firewall. The service contains a memory leak
   that is triggered by a particular type of malformed H.323 data.
   Each time such data is received, the memory available on the
   server is depleted by a small amount; if an attacker repeatedly
   sent such data, the performance of the server could deteriorate to
   the point where it would effectively disrupt all communications
   across the firewall. A server administrator could restore normal
   service by cycling the H.323 service. 
 - A denial of service vulnerability in the in the Proxy service.
   Like the vulnerability above, this one is caused by a memory leak,
   and could be used to degrade the performance of the server to
   the point where is disrupted communcations. 
 - A cross-site scripting vulnerability affecting the error page
   that ISA Server 2000 generates in response to a failed request
   for a web page. An attacker could exploit the vulnerability by
   tricking a user into submitting to ISA Server 2000 an URL that
   has the following characteristics: (a) it references a valid
   web site; (b)it requests a page within that site that can't be
   retrieved - that is, a non-existent page or one that generates
   an error; and (c) it contains script within the URL. The error
   page generated by ISA Server 2000 would contain the embedded
   script commands, which would execute when the page was displayed
   in the user's browser. The script would run in the security domain
   of the web site referenced in the URL, and would be able to access
   any cookies that site has written to the user's machine. 

Mitigating Factors:
====================
H.323 Denial of service vulnerability: 
 - The vulnerability could only be exploited if the H.323 Gatekeeper
   Service was installed. It is only installed by default if "Full
   Installation" is chosen; if "Typical Installation" is selected,
   it is not installed. 
 - The vulnerability would not enable an attacker to gain any
   privileges on an affected server or add any traffic to an existing
   voice-over-IP session. It is strictly a denial of service
   vulnerability. 

Proxy Service Denial of service vulnerability: 
 - The vulnerability could only be exploited by an internal user; it
   could not be exploited by an Internet user. 
 - The vulnerability would not enable an attacker to gain any
   privileges on an affected server or compromise any cached content
   on the server. It is strictly a denial of service vulnerability. 

Cross-site scripting vulnerability: 
 - In order to run script in the security domain of a trusted site,
   the attacker would need to know which sites, if any, a user
   trusted. Most users use the default security settings for all web
   sites, which would effectively deny an attacker any gain in
   exploiting the vulnerability for the purposes of running script. 
 - An attacker who wished to read other sites' cookies on a user's
   machine would have no way to know which sites had placed cookies
   there. The attacker would need to exploit the vulnerability once
   for every web site whose cookies she wished to access. 
 - Even if the attacker correctly guessed which sites had placed
   cookies on a user's machine, there should be no sensitive
   information in the cookies, if best practices have been followed. 

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin
   http://www.microsoft.com/technet/security/bulletin/ms01-045.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - Peter Grundl for reporting the memory leaks in the H.323
   Gatekeeper Service and the Proxy Service. 
 - Dr. Hiromitsu Takagi for reporting the cross-site scripting
   vulnerability. 

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