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CERT Advisory CA-2000-07 Microsoft Office 2000 UA ActiveX Control
Incorrectly Marked "Safe for Scripting"

Original release date: May 24, 2000
Last revised: --
Source: CERT/CC

A complete revision history is at the end of this file.

Systems Affected

* Systems with Internet Explorer and Microsoft Office 2000
components, including

* Word 2000
* Excel 2000
* PowerPoint 2000
* Access 2000
* Photodraw 2000
* FrontPage 2000
* Project 2000
* Outlook 2000
* Publisher 2000
* Works 2000 Suite


The Microsoft Office 2000 UA ActiveX control is incorrectly marked as
"safe for scripting". This vulnerability may allow an intruder to
disable macro warnings in Office products and, subsequently, execute
arbitrary code. This vulnerability may be exploited by viewing an HTML
document via a web page, newsgroup posting, or email message.

I. Description

Microsoft and L0pht Research Labs have recently published advisories
describing a vulnerability in the Microsoft Office 2000 UA ActiveX
control. Due to the severity of this vulnerability, we are issuing a
CERT advisory to help reach as broad an audience as possible.

ActiveX Overview

ActiveX controls are highly portable Component Object Model (COM)
objects, used extensively throughout Microsoft Windows platforms, and
especially in web-based applications. COM objects, including ActiveX
controls, can invoke each other through interfaces defined by the COM
architecture. The COM architecture allows for interoperability among
binary software components produced in disparate ways.

ActiveX controls can also be invoked from web pages through the use of
a scripting language or directly with an OBJECT tag. If an ActiveX
control is not installed locally, it is possible to specify a URL
where the control can be obtained. Once obtained, the control installs
itself automatically if permitted by the browser. Once it is
installed, it can be invoked without the need to be downloaded again.

ActiveX controls can be signed or unsigned. A signed control provides
a high degree of verification that the control was produced by the
signer and has not been modified. Signing does not guarantee the
benevolence, trustworthiness, or competence of the signer; it only
provides assurance that the control originated from the signer.

ActiveX controls are binary code capable of taking any action that the
user can take. They do not run in a "sandbox" of any kind. Because of
this, it is important to have a high degree of trust in the author of
the control. The CERT/CC recommends against installing any unsigned

Controls can also be marked as "safe for scripting" indicating that is
is permissible to invoke the control from a script contained in a web
page, using data and parameters provided by that page. In essence, a
control marked "safe for scripting" is an assertion by the author that
the control has implemented its own "sandbox" and cannot be used by an
intruder to damage or compromise your system. Because you must rely on
the author of the control to implement this "sandbox" correctly,
controls marked as "safe for scripting" require an especially high
degree of trust.

ActiveX controls are managed by the Windows registry, and it is
cumbersome to audit them or examine their properties without the use
of a specialized tool. One such tool is the OLE/COM Object Viewer
(oleview.exe) included with the Windows NT Resource Kit. More
information on oleview is available at

More information about ActiveX and COM can be found at

The Microsoft Office 2000 UA ActiveX Control

The UA ActiveX control implements the "Show Me" feature of the
interactive help system. Because the control is incorrectly marked
"safe for scripting", a malicious web author may use the UA ActiveX
control to script interactions that result in reduced security, such
as activating the dialog box for "Macro Security Setting" and
selecting the least secure choice. The control is correctly signed by

Other Advisories and Information

L0pht Research Labs and @Stake Inc. published an advisory describing
this vulnerability. They also produced a proof-of-concept exploit.
These documents are available from the L0pht web site:

Microsoft has published a security bulletin, an FAQ, and a
knowledgebase article describing this vulnerability. These documents
are available from Microsoft's web site:

II. Impact

The Office 2000 UA control is able to perform a wide variety of
actions within the Microsoft Office Product Suite, including
* Launch Internet Explorer
* Launch Microsoft Outlook
* Launch Microsoft Visual Basic
* Disable macro virus protection
* Save files

Perhaps the most significant impact is the ability to set Macro Virus
Protection to "Low", disabling warnings about malicious macro activity
in future documents. An intruder can exploit this vulnerability to
disable these warnings and then link directly to another Office
document that contains malicious macros. The macros in the second
document will run without confirmation and may take essentially any
action desired by the intruder.

Calls to the vulnerable control may originate in script or OBJECT tags
in web pages, newsgroup postings, or email messages.

As suggested by L0pht, this virus could be incorporated into an
electronic mail virus such as LoveLetter or Melissa. Note that
exploitation of this vulnerability under the default configuration of
Internet Explorer 5 and Microsoft Outlook 2000 does not require the
user to open any attachments or confirm any warning dialogs.

III. Solution

Apply a patch

Microsoft has produced a patch to correct this vulnerability. The
patch sets the "kill bit" for the vulnerable control and installs a
new control. The new control is similar to the original but lacks the
dangerous functionality. The new control is also marked "safe for

As a result of the removed functionality, the "Show Me" and "pop-up"
features of Office help will no longer function.

The patch is available through Office Update at

Limit Exposure to Vulnerability via Email

Since many e-mail applications provide the ability to start your web
browser automatically, you may wish to reduce your exposure via mail
messages by disabling scripting languages in your email client.

The Restricted Zone and Active Scripting

Microsoft suggests in their advisory to configure Outlook to view mail
in the Restricted Zone. While this is certainly good advice, it is not
sufficient to protect you from exploitation of this vulnerability if
the patch for the Office 2000 UA control has not been applied.

Because the Restricted Zone still allows the execution of scripts, an
intruder can send you an email message which when viewed starts
Internet Explorer and immediately exploits the vulnerability. To
protect against this scenario, and others like it, you may wish to
disable Active Scripting in the Restricted Zone.

Instructions for changing Outlook to use the Restricted Zone are
available in Microsoft's FAQ on this topic. Instructions for disabling
Active Scripting in the Restricted Zone are similar to those at

Note that these changes may result in reduced functionality in
Internet Explorer and Outlook.

Microsoft Outlook Security Update

Installing the Microsoft Outlook 2000 E-Mail Security Update will
modify Outlook to use the Restricted Zone as suggested previously. It
also limits which attachment file types are displayed in Outlook
messages, and adds new prompts for accessing the address book or
sending email messages. While none of these changes will protect you
completely from the Office 2000 UA vulnerability described in this
advisory, the update may significantly reduce the chance of the
vulnerability being exploited successfully on your system by a worm
propagating via Outlook.

More information about the Outlook 2000 E-Mail Security Update is
available from

Other Email Clients

If you use Internet Explorer as your web browser, you may wish to
disable JavaScript or other scripting languages in your email client
to prevent an email message from starting IE and exploiting this

Appendix A. Vendor Information

Microsoft Corporation

Microsoft has published a security bulletin, an FAQ, and a
knowledgebase article describing this vulnerability. These documents
are available from Microsoft's web site:

The CERT Coordination Center thanks L0pht Research Labs and @Stake for
initially discovering and reporting this vulnerability. We also thank
the Microsoft Security Team for their assistance in preparing this

Cory Cohen and Shawn Hernan were the primary authors of this document.

This document is available from:

CERT/CC Contact Information

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
email to and include SUBSCRIBE
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
May 24, 2000: Initial release

Version: PGP for Personal Privacy 5.0
Charset: noconv


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