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Title:      Downloaded Applications Can Execute on Mac IE 5.1 for 
            OS X
Date:       23 October 2001
Software:   Internet Explorer 5.1 for Macintosh (r)
Impact:     Run code of attacker's choice
Bulletin:   MS01-053

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-053.asp.
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Issue:
======
The Macintosh OS X Operating System provides built-in support for
both BinHex and MacBinary file types. These file types allow for the
efficient transfer of information across networks by allowing
information to be compressed by the sender and then decompressed by
the recipient. This capability is particularly useful on the
Internet, by allowing users to dowload compressed files. 

A vulnerability results because of a flaw in the way Mac OS X and Mac
IE 5.1 interoperate when BinHex and MacBinary file types are
downloaded. As a result, an application that is downloaded in either
of these formats can execute automatically once the download is
complete. 

A user would first have to choose to download a file and allow the
download to fully complete before the application could execute.
Also, users can choose to disable the automatic decoding of both
these file types. 

Mitigating Factors:
====================
 - The user would have to choose to downoad the application before 
   any attempt could be made to exploit the vulnerablity. It cannot
be 
   exploited without user interaction. 
 - The application would have to successfully download before any 
   attempt could be made to exploit the vulnerability. The user can 
   cancel the download at anytime prior to completion. 
 - The vulnerability could not be exploited if automatic decoding of 
   BinHex and MacBinary files has been disabled. This is not a
default
   setting however.

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms01-053.asp
   for information on obtaining this patch.


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