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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For security-related information about Microsoft products, please visit the Microsoft Security Advisor web site at http://www.microsoft.com/security.