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Title:      Cumulative Patch for Internet Explorer (810847)
Date:       05 February 2003
Software:   Microsoft Internet Explorer
Impact:     Allow an attacker to execute commands on a user's 
system. 
Max Risk:   Critical
Bulletin:   MS03-004

Microsoft encourages customers to review the Security Bulletins at: 
http://www.microsoft.com/technet/security/bulletin/ms03-004.asp
http://www.microsoft.com/security/security_bulletins/ms03-004.asp
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Issue:
======
This is a cumulative patch that includes the functionality of all 
previously released patches for IE 5.01, 5.5, 6.0. In addition, it 
eliminates two newly discovered vulnerabilities involving Internet 
Explorer's cross-domain security model - which keeps windows of 
different domains from sharing information. These flaws results in 
Internet Explorer because incomplete security checking causes 
Internet Explorer to allow one website to potentially access 
information from another domain when using certain dialog boxes. 

In order to exploit this flaw, an attacker would have to host a 
malicious web site that contained a web page designed to exploit this
particular vulnerability and then persuade a user to visit that site.
Once the user has visited the malicious web site, it would be 
possible for the attacker to run malicious script by misusing a 
dialog box and cause that script to access information in a different
domain. In the worst case, this could enable the web site operator to
load malicious code onto a user's system. In addition, this flaw 
could also enable an attacker to invoke an executable that was 
already present on the local system. 

A related cross-domain vulnerability allows Internet Explorer's 
showHelp() functionality to execute without proper security 
checking. showHelp() is one of the help methods used to display an 
HTML page containing help content. showHelp() allows more types of 
pluggable protocols than necessary, and this could potentially allow 
an attacker to access user information, invoke executables already 
present on a user's local system or load malicious code onto a user's
local system. 

The requirements to exploit this vulnerability are the same as for 
the issue described above: an attacker would have to host and lure a 
user to a malicious web site. In this scenario, the attacker could 
open a showHelp window to a known local file on the visiting user's 
local system and gain access to information from that file by sending
a specially crafted URL to a second showHelp window. The attacker 
could also potentially access user information or run code of 
attacker's choice. 

This cumulative patch will cause window.showHelp( ) to cease to 
function. When the latest HTML Help update - which is being released 
via Windows Update with this patch - is installed, window.showHelp( )
will function again, but with some limitations (see the caveats 
section later in this bulletin). This has been necessary in order to 
block the attack vector that might allow a web site operator to 
invoke an executable that was already present on a user's local 
system. 

Mitigating Factors:
====================
 - The attacker would have to host a web site that contained a web 
page used to exploit either of these cross-domain vulnerabilities. 
 - The attacker would have no way to force users to visit the site. 
Instead, the attacker would need to lure them there, typically by 
getting them to click on a link that would take them to the 
attacker's site. 
 - By default, Outlook Express 6.0 and Outlook 2002 open HTML mail 
in the Restricted Sites Zone. In addition, Outlook 98 and 2000 open 
HTML mail in the Restricted Sites Zone if the Outlook Email Security 
Update has been installed. Customers who use any of these products 
would be at no risk from an e-mail borne attack that attempted to 
exploit this vulnerability unless the user clicked a malicious link 
in the email. 
 - Internet Explorer 5.01 users are not affected by the first 
vulnerability. 
 
Risk Rating:
============
 - Internet Explorer 5.01: Critical
 - Internet Explorer 5.5: Critical
 - Internet Explorer 6.0: Critical

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletins at

   http://www.microsoft.com/technet/security/bulletin/ms03-004.asp
   http://www.microsoft.com/security/security_bulletins/ms03-004.asp

   for information on obtaining this patch.

Acknowledgment:
===============
 - Andreas Sandblad, Sweden for reporting the cross domain 
vulnerability using showhelp.

Home User Security Notification Service
=======================================
Microsoft is now offering the Microsoft Security Update, a security 
bulletin notification service for home users.  To learn more about 
this service, please go to: 

http://www.microsoft.com/security/security_bulletins/decision.asp

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