Microsoft Security Bulletin MS03-020: Cumulative Patch for Internet Explorer (818529)

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Title:      Cumulative Patch for Internet Explorer (818529)
Date:       04 June 2003
Software:   Microsoft(r) Microsoft Internet Explorer(r) 5.01 
            Microsoft Internet Explorer 5.5 
            Microsoft Internet Explorer 6.0 
            Microsoft Internet Explorer 6.0 for Windows Server 2003 
Impact:     Allow an attacker to execute code of their choice
Max Risk:   Critical
Bulletin:   MS03-020

Microsoft encourages customers to review the Security Bulletins at: 
http://www.microsoft.com/technet/security/bulletin/MS03-020.asp
http://www.microsoft.com/security/security_bulletins/ms03-020.asp
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Issue:
======
This is a cumulative patch that includes the functionality of all 
previously released patches for Internet Explorer 5.01, 5.5 and 6.0. 
In addition, it eliminates two newly discovered vulnerabilities: 


 - A buffer overrun vulnerability that occurs because Internet 
Explorer does not properly determine an object type returned from a 
web server. It could be possible for an attacker who exploited this 
vulnerability to run arbitrary code on a user's system. If a user 
visited an attacker's website, it would be possible for the attacker 
to exploit this vulnerability without any other user action. An 
attacker could also craft an HTML email that attempted to exploit 
this vulnerability. 

 - A flaw that results because Internet Explorer does not implement 
an appropriate block on a file download dialog box. It could be 
possible for an attacker to exploit this vulnerability to run 
arbitrary code on a user's system. If a user simply visited an 
attacker's website, it would be possible for the attacker to exploit 
this vulnerability without any other user action. An attacker could 
also craft an HTML email that attempted to exploit this 
vulnerability. 

In order to exploit these flaws, the attacker would have to create a 
specially formed HTML email and send it to the user. Alternatively 
an attacker would have to host a malicious web site that contained a 
web page designed to exploit these vulnerabilities. The attacker 
would then have to persuade a user to visit that site. 

As with the previous Internet Explorer cumulative patches released 
with bulletins MS03-004 and MS03-015, this cumulative patch will 
cause window.showHelp( ) to cease to function if you have not 
applied the HTML Help update. If you have installed the updated HTML 
Help control from Knowledge Base article 811630, you will still be 
able to use HTML Help functionality after applying this patch.

Mitigating factors: 
====================
The following mitigating factors apply to both vulnerabilities 
discussed in this bulletin:


 - By default, Internet Explorer on Windows Server 2003 runs in 
Enhanced Security Configuration. This default configuration of 
Internet Explorer blocks these attacks. If Internet Explorer 
Enhanced Security Configuration has been disabled, the protections 
put in place that prevent these vulnerabilities from being exploited 
would be removed. 
 - In the Web based attack scenario, the attacker would have to host 
a web site that contained a web page used to exploit these 
vulnerabilities. An attacker would have no way to force users to 
visit a malicious web site outside of the HTML email vector. 
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. 
 - Code that executed on the system would only run under the 
privileges of the logged in user.  

Risk Rating:
============
Critical

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletins at
   
http://www.microsoft.com/technet/security/bulletin/ms03-020.asp
http://www.microsoft.com/security/security_bulletins/ms03-020.asp
   
   for information on obtaining this patch.

Acknowledgment:
===============
 - eEye Digital Security (http://www.eeye.com/)

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