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AUSCERT External Security Bulletin Redistribution
ESB-2000.184 -- CERT Advisory CA-2000-14
Microsoft Outlook and Outlook Express
Cache Bypass Vulnerability
27 July 2000
AusCERT Security Bulletin Summary
Product: Outlook Express 4.0 to 5.01
Operating System: MS Windows
Impact: Read and Create Arbitrary files
Access Required: Remote
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CERT Advisory CA-2000-14 Microsoft Outlook and Outlook Express Cache Bypass
Original release date: July 26, 2000
A complete revision history is at the end of this file.
* 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
"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."
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.
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:
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Copyright 2000 Carnegie Mellon University
July 26, 2000: Initial release
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