Microsoft Security Bulletin MS01-057 (version 2.0)

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Title:      Specially Formed Script in HTML Mail can Execute in
            Exchange 5.5 OWA
Date:       06 December 2001
Revised:    07 December 2001 (version 2.0)
Software:   Microsoft Exchange 5.5 Server Outlook Web Access
Impact:     Run Code of Attacker's Choice
Max Risk:   Medium
Bulletin:   MS01-057

Microsoft encourages customers to review the Security Bulletin at: 
http://www.microsoft.com/technet/security/bulletin/MS01-057.asp.
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Reason for Revision:
====================
On December 6, 2001 Microsoft released the original version of this
bulletin. On December 7, 2001 an issue relating to file dependencies
for the patch was identified and the bulletin was updated and
re-released
to include this information. Specifically, for this patch to function
properly, the Outlook Web Access (OWA) server on which the patch is
installed must have Internet Explorer (IE) 5.0 or greater installed.
If
the patch is installed on a system with a version of IE older than
5.0,
unexpected consequences may result. The "Caveats" section has been
updated to include version requirements for this patch. In addition,
it
contains version recommendations for dependent components that are
applicable at the time of this writing. In addition, the FAQ contains
remediation information for customers who have applied this patch
on systems with versions of IE older than 5.0. 

Issue:
======
Outlook Web Access (OWA) is a service of Exchange 5.5 Server that
allows users to access and manipulate messages in their Exchange
mailbox by using a web browser. 

A flaw exists in the way OWA handles inline script in messages in
conjunction with Internet Explorer (IE). If an HTML message that
contains specially formatted script is opened in OWA, the script
executes when the message is opened. Because OWA requires that
scripting be enabled in the zone where the OWA server is located,
a vulnerability results because this script could take any action
against the user's Exchange mailbox that the user himself was
capable of, including sending, moving, or deleting messages. An
attacker could maliciously exploit this flaw by sending a 
specially crafted message to the user. If the user opened the
message in OWA, the script would then execute. 

While it is possible for a script to send a message as the user,
it is impossible for the script to send a message to addresses in
the user's address book. Thus, the flaw cannot be exploited for
mass-mailing attacks. Also, mounting a successful attack requires
knowledge of the intended victim's choice of mail clients and
reading habits. If the maliciously crafted message were read in
any mail client other than a browser through OWA, the attack
would fail. 

Mitigating Factors:
====================
 - A successful attack would require the victim to read the message
   in a IE using OWA only. The attack would fail if read in any
   other mail client. 
 - A successful attack would also require knowledge of the version
   of OWA in use. The attack would fail on other versions of OWA. 
 - A successful attack can only take action on the mailbox on the 
   Exchange Server as the user. It cannot take action on the user's
   local machine. It cannot take actions on any other users mailbox
   directly. Nor can it take actions directly on the Exchange Server.

Risk Rating:
============
 - Internet systems: Moderate
 - Intranet systems: Moderate
 - Client systems: None

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms01-057.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - Lex Arquette of WhiteHat Security (http://www.whitehatsec.com)
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