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Title:      Windows Media Player .ASX Processor Contains Unchecked
            Buffer
Date:       23 May 2001
Software:   Windows Media Player 6.4 and 7
Impact:     Potentially run code of attacker's choice.
Bulletin:   MS01-029

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-029.asp.
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Issue:
======
This bulletin discusses two security vulnerabilities that are related
to each other only by the fact that they affect Windows Media Player.
We packaged them in a single patch for customers using Windows Media
Player 6.4 to make it more convenient for customers to apply. For
customers using Windows Media Player 7, both security vulnerabilities
are addressed by upgrading to Windows Media Player 7.1.

The two vulnerabilities are:

 - A buffer overrun in the functionality used to process Active
Stream
Redirector (.ASX) files. This vulnerability is a variant of the
buffer
overrun vulnerability identified in Microsoft Security Bulletin
(MS00-090). Windows Media Player supports the use of .ASX files to
enable users to play streaming media that resides on intranet or
Internet sites and allows the use of playlists. However, the code
that parses .ASX files has an unchecked buffer, and this could
potentially enable a malicious user to run code of her choice on the
machine of another user. The attacker could either send an affected
file to another user and entice him to run or preview it, or she
could
host such a file on a web site and cause it to launch automatically
whenever a user visited the site. The code could take any action on
the machine that the legitimate user himself could take. 

 - A vulnerability affecting how Windows Media Player handles
Internet
shortcuts. Windows Media Player has a flaw that causes it to save
Internet shortcuts to the user's Temporary Files folder with a fixed
known filename. This results in a security vulnerability because it's
possible for HTML code to be stored in such a shortcut and launched
via
a web page or HTML e-mail, in which case the code would run in the
Local Computer Zone rather than the Internet Zone. An attacker could
exploit this vulnerability to read - but not add, delete or modify -
files on another user's computer. 

 - In addition, this patch provides a solution to a potential privacy
vulnerability that was recently identified. This issue could be 
exploited by a malicious set of web sites to distinguish a user.
While 
this issue would not by itself enable a web site to identify the
user, 
it could enable the correlation of user information to potentially 
build a composite description of the user. .Users can protect 
themselves by installing the above patch or upgrading to Windows
Media 
Player 7.1, then changing the appropriate settings in their player as
outlined below to prevent sets of websites from potentially profiling
using Windows Media Player.


 - In Windows Media Player 6.4, the privacy setting is selected via a
new option, which can be reached by going to the menu item View / 
Options then selecting the player tab and de-selecting "Allow
Internet 
sites to uniquely identify your player". 

 - In Windows Media Player 7.1, the privacy setting is toggled via
the 
existing option under the tools menu, on the player tab and deselect 
the option "Allow Internet sites to uniquely identify your player". 

Although we typically do not discuss privacy issues in security 
bulletins, the privacy issue in this case is eliminated by applying
the 
patch and then selecting the new user settings as described above. We
have provided this information because the best way to make the
privacy 
update available to customers was by including it in this patch, and 
because we wanted to provide users who installed the patch with 
information about how to use the new privacy settings. 

Mitigating Factors:
====================
Buffer overrun vulnerability:

 - The attacker would need the ability to entice the user into either
visiting a web site she controlled, or opening an HTML e-mail she had
prepared. 

 - The attacker would need to know the specific operating system that
the user was running in order to tailor the attack code properly; if 
the attacker made an incorrect guess about the user's operating
system 
platform, the attack would crash the user's application, but not run 
code of the attacker's choice. 

Internet shortcut vulnerability: 

 - On Windows NT 4.0 and Windows 2000 systems, the location of the 
Temporary Files folder varies from user to user. In order to exploit 
the vulnerability on these systems, the attacker would need to know
the 
exact location of the Temporary Files folder on the specific system
she 
wished to attack. 

 - The attacker would need to know the exact name of each file she 
wished to read. 

 - The attacker could only view file types that can be opened in a 
browser window. These include.txt, .jpg, .gif, or .htm , but not file
types such as .exe, .doc, and .xls. 

 - There is no capability to add, delete or changes files via this 
vulnerability. 

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin
   http://www.microsoft.com/technet/security/bulletin/ms01-029.asp
   for information on obtaining this patch.

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