Software Engineering Institute

Microsoft COM

Microsoft COM is a technology that allows programmers to create reusable software components that can be incorporated into applications to extend their functionality. Microsoft COM includes COM+, Distributed COM (DCOM), and ActiveX Controls.

ActiveX controls

ActiveX controls are COM objects that are designed to be used in Internet Explorer. A web page can make use of an ActiveX control through several means, such as by using an OBJECT tag.

Kill bit

If a security flaw is discovered in an ActiveX control, the control may be disabled in Internet Explorer. This is accomplished by setting the "kill bit" for the control, as described in Microsoft Knowledge Base article 240797. Before instantiating an ActiveX control, Internet Explorer will check the value of the Compatibility Flags registry entry. If the value is DWORD 00000400, Internet Explorer will not use the control.

The problem

A specially crafted HTML document can cause Internet Explorer to skip the kill bit check. This means that any ActiveX control that has been disabled solely through use of the kill bit may still be used by Internet Explorer.

Note that this vulnerability is unrelated to VU#959049 - Multiple COM objects cause memory corruption in Microsoft Internet Explorer.

Depending on which control an attacker uses, the impact will vary. By convincing a user to view a specially crafted HTML document (e.g., a web page or an HTML email message), an attacker could execute arbitrary code with the privileges of the user (e.g., VU#29795, VU#939605). An attacker may also be able to create or edit arbitrary files (e.g., VU#9162, VU#23412), access local configuration data (e.g. VU#1673), or take other actions.

Apply an update
Install the 905915 update (MS05-054) or a more recent Internet Explorer cumulative security update. The MS05-054 update improves the way that Internet Explorer checks the kill bit.

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 document Securing Your Web Browser and the Malicious Web Scripts FAQ.

Note that disabling ActiveX controls in the Internet Zone will reduce the functionality of some web sites.

Use a different web browser

There are a number of significant vulnerabilities in technologies involving the IE domain/zone security model, local file system (Local Machine Zone) trust, the Dynamic HTML (DHTML) document object model (in particular, proprietary DHTML features), the HTML Help system, MIME type determination, the graphical user interface (GUI), and ActiveX. These technologies are implemented in operating system libraries that are used by IE and many other programs to provide web browser functionality. IE is integrated into Windows to such an extent that vulnerabilities in IE frequently provide an attacker significant access to the operating system.

It is possible to reduce exposure to these vulnerabilities by using a different web browser, especially when viewing untrusted HTML documents (e.g., web sites, HTML email messages). Such a decision may, however, reduce the functionality of sites that require IE-specific features such as proprietary DHTML, VBScript, and ActiveX. Note that using a different web browser will not remove IE from a Windows system, and other programs may invoke IE, the WebBrowser ActiveX control (WebOC), or the HTML rendering engine (MSHTML).