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AUSCERT External Security Bulletin Redistribution
ESB-2001.134 -- CERT Advisory CA-2001-06
Automatic Execution of Embedded MIME Types
5 April 2001
AusCERT Security Bulletin Summary
Product: Microsoft Internet Explorer
Impact: Execute Arbitrary Code/Commands
Access Required: Remote
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CERT Advisory CA-2001-06 Automatic Execution of Embedded MIME Types
Original release date: April 03, 2001
Last revised: --
A complete revision history can be found at the end of this file.
* All versions of Microsoft Internet Explorer 5.5 SP1 or earlier,
except IE 5.01 SP2
* Any software which utilizes vulnerable versions of Internet
Explorer to render HTML
Microsoft Internet Explorer has a vulnerability triggered when parsing
MIME parts in a document that allows a malicious agent to execute
arbitrary code. Any user or program that uses vulnerable versions of
Internet Explorer to render HTML in a document (for example, when
browsing a filesystem, reading email or news messages, or visiting a
web page), should immediately upgrade to a non-vulnerable version of
There exists in Internet Explorer a table which is used to determine
how IE handles MIME types when it encounters MIME parts in any type of
HTML document, be it email message, newsgroup posting, web page, or
local file. This table contains a set of entries that cause Internet
Explorer to open the MIME part without giving the end user the
opportunity to decide if the MIME part should be opened. This
vulnerability allows an intruder to construct malicious content that,
when viewed in Internet Explorer (or any program that uses the IE HTML
rendering engine), can execute arbitrary code. It is not necessary to
run an attachment; simply viewing the document in a vulnerable program
is sufficient to execute arbitrary code.
For more details, see Microsoft Security Bulletin MS01-020 on this
There have been reports that simply previewing HTML content (as in a
mail client or filesystem browser) is sufficient to trigger the
vulnerability. The impact of viewing malicious code in this manner is
The CERT/CC is currently unaware of any reports of this vulnerability
being used to successfully attack a system. Demonstration code
exploiting this vulnerability has been published in several public
forums. This vulnerability is being referenced in CVE as CAN-2001-0154
and by the CERT/CC as VU#980499.
Attackers can cause arbitrary code to be executed on a victim's system
by embedding the code in a malicious email, or news message, or web
Apply the patch from Microsoft
Apply the patch from Microsoft, available at:
As noted in the 'Caveats' section of the Microsoft advisory, end users
must apply this patch to supported versions of Microsoft's browser.
This means IE must be upgraded to IE 5.01 Service Pack 1 or IE 5.5
Service Pack 1 before users can apply this patch. Users who have not
previously upgraded will incorrectly receive a message stating that
they do not need to apply this patch, even though they are vulnerable.
Users are advised to upgrade to IE 5.5 SP1, IE 5.01 SP1 or SP2 (which
has this patch incorporated in it) and apply the appropriate patch.
An excerpt from MS01-020:
If the patch is installed on a system running a version of IE other
than the one it is designed for, an error message will be displayed
saying that the patch is not needed. This message is incorrect, and
customers who see this message should upgrade to a supported version
of IE and re-install the patches.
Appendix A. - Vendor Information
This appendix contains information provided by vendors for this
advisory. When vendors report new information to the CERT/CC, we
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their
Cyrusoft International, Inc.
Mulberry does not use Internet Explorer to render HTML within Mulberry
itself and is not vulnerable to these kinds of problems. Users can
save HTML attachments to disk and then view those in browsers
susceptible to this problem, but this requires the direct intervention
of the user to explicitly save to disk - simply viewing HTML in
Mulberry does not expose users to these kinds of problems.
Our HTML rendering is a basic styled-text only renderer that does not
execute any form of scripts. This is true on all the platforms we
support: Win32, Mac OS (Classic & X), Solaris, linux.
An official statement about this is available on our website at:
Lotus Development Corporation
Notes does not use IE to render HTML-formatted mail messages.
Please see the advisory (MS01-020, "Incorrect MIME Header Can Cause IE
to Execute E-mail Attachment") related to this issue at:
A patch is available for this issue at:
Netscape Communications Corporation
Netscape is currently investigating the impact this vulnerability, if
any, has on users of the Netscape browser.
Opera does not use Internet Explorer or any other external software to
It is unclear at this time what impact, if any, this vulnerability has
on Eudora clients.
Appendix B. - References
1. Havrilla, J., and Hernan, S., "CERT Vulnerability Note VU#980499:
Certain MIME types can cause Internet Explorer to execute
arbitrary code when rendering HTML", March 2001.
Microsoft has acknowledged Juan Carlos Cuartango for bringing this
issue to their attention.
This document was written by Jeffrey S. Havrilla and Shawn V. Hernan.
If you have feedback, comments, or additional information about this
issue, please send us email.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
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Our public PGP key is available from
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Conditions for use, disclaimers, and sponsorship information
Copyright 2001 Carnegie Mellon University.
April 03, 2001: Initial release
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