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CERT Advisory CA-2000-14 Microsoft Outlook and Outlook Express Cache Bypass

   Original release date: July 26, 2000
   Source: CERT/CC
   A complete revision history is at the end of this file.
Systems Affected

     * Any system running Microsoft Outlook Express 4.0 or 4.01
     * Any system running Microsoft Outlook Express 5.0 or 5.01
     * Any system running Microsoft Outlook 98
     * Any system running Microsoft Outlook 2000

   Microsoft has recently released Microsoft Security Bulletin MS00-046,
   in which they announced a patch for the "Cache Bypass" vulnerability.
   By exploiting this vulnerability, an attacker can use an
   HTML-formatted message to read certain types of files on the victim's
   In addition, because this vulnerability also allows the attacker to
   store files on the victim's machine, it can be used in conjunction
   with existing vulnerabilities to execute arbitrary code on the target
I. Description

"Cache Bypass" Vulnerability

   Typically, all files downloaded by either Outlook or Internet Explorer
   are stored in an area known as a cache. The cache serves two main
   purposes. First, it provides temporary storage for online content,
   which minimizes the amount of data that must be transferred when
   refreshing a page. Second, it provides an area where Internet content
   can be downloaded to the local machine and accessed with the same
   security policy as remote content.
   This vulnerability allows attackers to use an HTML-formatted message
   to store files outside the cache. Inside the cache, the files are
   governed by the security policy of the "Internet Zone," but outside
   they are governed by the "Local Computer Zone." Once a file is stored
   in the "Local Computer Zone," the security policy of the "Internet
   Zone" no longer applies to it. This could put systems at risk because
   the security policies of the "Local Computer Zone" are typically more
   permissive than those of the "Internet Zone."
II. Impact

   When exploited, this vulnerability allows an attacker to store an HTML
   file in an area that is not protected by the policies of the "Internet
   Zone." This file may then be used to open arbitrary files on the
   victim's machine and send their contents back to the attacker.
   In addition, the "Cache Bypass" vulnerability could be used in
   conjunction with other vulnerabilities to allow an intruder to execute
   arbitrary code on the victim's machine.
III. Solution

   Microsoft has released Microsoft Security Bulletin MS00-046, which
   points to a patch for this vulnerability. We strongly encourage you to
   read this bulletin and apply the patch. MS00-046 is available at
   The CERT Coordination Center would like to thank Microsoft for its
   assistance in developing this advisory.
   Author: Jeffrey P. Lanza
   This document is available from:
CERT/CC Contact Information

   Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
          Postal address:
          CERT Coordination Center
          Software Engineering Institute
          Carnegie Mellon University
          Pittsburgh PA 15213-3890
   CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) / EDT(GMT-4)
   Monday through Friday; they are on call for emergencies during other
   hours, on U.S. holidays, and on weekends.
Using encryption

   We strongly urge you to encrypt sensitive information sent by email.
   Our public PGP key is available from
   If you prefer to use DES, please call the CERT hotline for more
Getting security information

   CERT publications and other security information are available from
   our web site
   To be added to our mailing list for advisories and bulletins, send
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   your-email-address in the subject of your message.
   * "CERT" and "CERT Coordination Center" are registered in the U.S.
   Patent and Trademark Office.
   Any material furnished by Carnegie Mellon University and the Software
   Engineering Institute is furnished on an "as is" basis. Carnegie
   Mellon University makes no warranties of any kind, either expressed or
   implied as to any matter including, but not limited to, warranty of
   fitness for a particular purpose or merchantability, exclusivity or
   results obtained from use of the material. Carnegie Mellon University
   does not make any warranty of any kind with respect to freedom from
   patent, trademark, or copyright infringement.
   Conditions for use, disclaimers, and sponsorship information
   Copyright 2000 Carnegie Mellon University
   Revision History
July 26, 2000:  Initial release

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