OVERVIEW
========

The method used for Windows security zone evaluation fails when 
characters in the URL are encoded in a certain way. Internet Explorer 
can be tricked to think that a document belongs in "My Computer" zone 
when it actually resides on an Internet server. JavaScript in such 
document can be used to execute arbitrary code because documents in "My 
Computer" zone are normally trusted and given more privileges than 
documents on Internet.

A malicious user can use this vulnerability to do any action on the 
victim system with the victim user's privileges - transfer files, run 
programs, etc. No further user interaction is required apart from 
viewing a web page created by the attacker. In the e-mail attack 
scenario the victim user is usually required to click a link in the 
e-mail.



DETAILS
=======

Somewhere in the process of evaluating the security zone for URLs,
hex-decoding (the %xy notation) is done more than once for a single 
URL, ie. the decoded URL is decoded again. This causes some undesired 
effects if the URL contains certain special characters multiply 
encoded.

Unlike some other operating systems, Windows allows the % sign in 
hostnames, so a URL containing such encoding works in Internet Explorer 
- given that the hostname resolves correctly to the attacker's IP 
address. The attacker can then host e.g. an HTML document on the 
server, which Internet Explorer misinterprets as belonging in "My 
Computer" zone.



VULNERABLE VERSIONS
===================

A proof-of-concept exploit was tested with Internet Explorer 6 on 
Windows 2000 and Windows XP. The exploit successfully launches an 
attacker-supplied EXE program when the victim user visits a web page 
containing the exploit. A full list of vulnerable versions is included 
in Microsoft's bulletin (link below).



VENDOR STATUS
=============

Microsoft was informed of the problem on February 16th, 2004. A 
preliminary patch was first produced in September 2004 and Microsoft 
sent it to me for testing. However it turned out that the fix didn't 
correctly protect from a variation of the exploit, so the release was 
delayed.

The final patch and Microsoft's bulletin is available at

http://www.microsoft.com/technet/security/bulletin/ms05-014.mspx 



CREDITS
=======

The vulnerability was discovered and researched by Jouko Pynnönen, 
Finland.




-- 
Jouko Pynnönen Web: http://iki.fi/jouko/ 
jouko@iki.fi