Software Engineering Institute

ActiveX

ActiveX is a technology that allows programmers to create reusable software components that can be incorporated into applications to extend their functionality. Internet Explorer is a common Windows application that makes use of ActiveX controls.

ActiveX safety determination

Internet Explorer determines if an ActiveX control is safe by querying the IObjectSafety interface of the object and by querying the Implemented Categories registry key for the control, as specified by Microsoft Knowledge Base Article 216434 and the MSDN ActiveX safety article.

ActiveX security options

Through either the IObjectSafety interface or the appropriate registry values, an ActiveX control can be marked as "safe for scripting" and/or "safe for initialization." According to the MSDN article Signing and Marking ActiveX Controls:

If you mark your control as safe for initializing, you are asserting that no matter what values are used to initialize your control, it won't do anything that would damage a user's system or compromise the user's security.

If you mark your control as safe for scripting, you are asserting that your control won't do anything to damage a user's system or compromise the user's security, regardless of how your control's methods and properties are manipulated by the Web page's script. In other words, it has to accept any method calls (with any parameters) and/or property manipulations in any order without doing anything bad.
The MSDN article Designing Secure ActiveX Controls states:

Controls are marked as not safe for scripting or data initialization by default. Don't implement them unless the functionality of the control is hampered without them.
The WMI Object Broker ActiveX control

The WMI Object Broker ActiveX control is an object that comes with Microsoft Visual Studio 2005. It may also be present in other software packages. The control is provided by the file wmiscriptutils.dll. This control is marked as "safe for scripting," which means that a web page can control the object, such as by calling its methods.

The problem

The WMI Object Broker ActiveX control includes a method that can create an instance of an ActiveX control that exists on the system. The ActiveX objects created in this manner will bypass the ActiveX security model. For example, the "kill bit" and "safe for scripting" options are ignored.

Note that public reports indicate this vulnerability is actively being exploited.

For more information refer to Microsoft Security Advisory (927709).

By convincing a user to view a specially crafted HTML document (e.g., a web page or an HTML email message or attachment), an attacker may be able to execute arbitrary code with the privileges of the user.

Apply Updates

Apply the appropriate updates described in Microsoft Security Bulletin MS06-073. This update provides a newer version of the WMI Object Broker ActiveX control. Microsoft Security Bulletin MS07-016 also sets the kill bit for the WMI Object Broker ActiveX control, which prevents Internet Explorer from using it.

Disable the WMI Object Broker ActiveX control in Internet Explorer

The WMI Object Broker ActiveX control can be disabled in Internet Explorer by setting the kill bit for the following CLSID:

{7F5B7F63-F06F-4331-8A26-339E03C0AE3D}

More information about how to set the kill bit is available in Microsoft Support Document 240797.

Disable ActiveX

Disabling ActiveX controls in the Internet Zone (or any zone used by an attacker) appears to prevent exploitation of this vulnerability. Instructions for disabling ActiveX in the Internet Zone can be found in the "Securing Your Web Browser" document and the Malicious Web Scripts FAQ.