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

Original release date: May 24, 2000
Last revised: May 26, 2000
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

Overview

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.

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.

Controls can also be marked as "safe for scripting" indicating that it 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

http://www.microsoft.com/Com/resources/oleview.asp

More information about ActiveX and COM can be found at

http://www.microsoft.com/com

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 Microsoft.

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:

http://www.l0pht.com/advisories/msoua.txt

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

http://microsoft.com/technet/security/bulletin/ms00-034.asp

http://microsoft.com/technet/security/bulletin/fq00-034.asp

http://microsoft.com/technet/support/kb.asp?ID=262767

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.

III. Solution

Apply a patch

Microsoft has produced a patch to correct this vulnerability. The patch installs a new version of the control lacking the dangerous functionality. The new version is also marked "safe for scripting".

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

http://officeupdate.microsoft.com/info/ocx.htm

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

http://www.cert.org/tech_tips/malicious_code_FAQ.html#steps

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

http://www.officeupdate.com/2000/downloadDetails/Out2ksec.htm

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 vulnerability.

Appendix A. Vendor Information

Microsoft Corporation