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CA-2002-04 Buffer Overflow in Microsoft Internet Explorer


CERT Advisory CA-2002-04 Buffer Overflow in Microsoft Internet Explorer

Original release date: February 25, 2002
Last revised: --
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

* Microsoft Internet Explorer
* Microsoft Outlook and Outlook Express
* Other applications that use the Internet Explorer HTML rendering


Microsoft Internet Explorer contains a buffer overflow vulnerability
in its handling of embedded objects in HTML documents. This
vulnerability could allow an attacker to execute arbitrary code on the
victim's system when the victim visits a web page or views an HTML
email message.

I. Description

Internet Explorer supports the <EMBED> directive, which can be used to
include arbitrary objects in HTML documents. Common types of embedded
objects include multimedia files, Java applets, and ActiveX controls.
The SRC attribute specifies the source path and filename of an object.
For example, a MIDI sound might be embedded in a web page with the
following HTML code:

<EMBED TYPE="audio/midi" SRC="/path/sound.mid" AUTOSTART="true">

Internet Explorer uses attributes of the <EMBED> directive and MIME
information from the web server to determine how to handle an embedded
object. In most cases, a separate application or plugin is used.

A group of Russian researchers, SECURITY.NNOV, has reported that
Internet Explorer does not properly handle the SRC attribute of the
<EMBED> directive. An HTML document, such as a web page or HTML email
message, that contains a crafted SRC attribute can trigger a buffer
overflow, executing code with the privileges of the user viewing the
document. Microsoft Internet Explorer, Outlook, and Outlook Express
are vulnerable. Other applications that use the Internet Explorer HTML
rendering engine, such as Windows compiled HTML help (.chm) files and
third-party email clients, may also be vulnerable.

The CERT/CC is tracking this vulnerability as VU#932283, which
corresponds directly to the "buffer overrun" vulnerability described
in Microsoft Security Bulletin MS02-005.

This vulnerability has been assigned the CVE identifier CAN-2002-0022.

II. Impact

By convincing a user to view a malicious HTML document, an attacker
can cause the Internet Explorer HTML rendering engine to execute
arbitrary code with the privileges of the user who viewed the HTML
document. This vulnerability could be exploited to distribute viruses,
worms, or other malicious code.

III. Solution

Apply a patch

Microsoft has released a cumulative patch for Internet Explorer that
corrects this vulnerability and several others. For more information
about the patch and the vulnerabilities, please see Microsoft Security
Bulletin MS02-005:

Disable ActiveX Controls and Plugins

In Internet Explorer, plugins may be used to view, play, or otherwise
process embedded objects. The execution of embedded objects is
controlled by the "Run ActiveX Controls and Plugins" security option.
Disabling this option will prevent embedded objects from being
processed, and will therefore prevent exploitation of this

According to MS02-005:

The vulnerability could not be exploited if the "Run ActiveX
Controls and Plugins" security option were disabled in the Security
Zone in which the page was rendered. This is the default condition
in the Restricted Sites Zone, and can be disabled manually in any
other Zone.

At a minimum, disable the "Run ActiveX Controls and Plugins" security
option in the Internet Zone and the zone used by Outlook or Outlook
Express. The "Run ActiveX Controls and Plugins" security option is
disabled in the "High" zone security setting. Instructions for
configuring the Internet Zone to use the "High" zone security setting
can be found in the CERT/CC Malicious Web Scripts FAQ:

Apply the Outlook Email Security Update

Another way to effectively disable the processing of ActiveX controls
and plugins in Outlook is to install the Outlook Email Security
Update. The update configures Outlook to open email messages in the
Restricted Sites Zone, where the "Run ActiveX Controls and Plugins"
security option is disabled by default. In addition, the update
provides further protection against malicious code that attempts to
propagate via Outlook.

* Outlook 2002 and Outlook Express 6
The functionality of the Outlook Email Security Update is included
in Outlook 2002 and Outlook Express 6.
* Outlook 2000
* Outlook 98

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


Microsoft has released a Security Bulletin and a Knowledge Base
Article addressing this vulnerability:
* Security Bulletin MS02-005
* Knowledge Base Article Q317731;en-us;Q317731


Our email client Mulberry does not use the core HTML rendering engine
library for its HTML display, and so is not affected by the bug in
that library. Having looked at the details of this alert I can also
confirm that our own HTML rendering engine is not affected by this, as
it ignores the relevant tags.

Appendix B. - References



The CERT/CC thanks ERRor and DarkZorro of domain Hell and 3APA3A of
SECURITY.NNOV for reporting this issue to us.

Author: Art Manion

This document is available from:

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Copyright 2002 Carnegie Mellon University.

Revision History
February 25, 2002: Initial release

Version: PGP 6.5.8


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