Vulnerability Note VU#823260

Microsoft provides a shared HTML conversion library (html32.cnv) that is used by IE and other Windows applications. According to MS03-023, "The HTML converter is an extension which allows applications to convert HTML data into Rich Text Format (RTF) while maintaining the formatting and structure of the data as well as the text. The converter also supports the conversion of RTF data into HTML."

The HTML conversion library contains a buffer overflow that can be exploited when IE opens a specially crafted HTML document. In a publicly available example, script automates the process of creating a new HTML document and opening it in a frame off screen, writing a specially crafted align element in an <HR> tag to the document, selecting the contents of the document, copying the contents to the clipboard, and closing the frame. The library is loaded when the frame is closed and the crafted align element overflows a buffer on the stack, allowing the attacker to control the contents of the EIP register.

The known attack relies on IE and Active scripting. It is possible that other attack vectors exist. For example, Microsoft FrontPage, WordPad, and Office (Word, Excel, PowerPoint, Access) use the vulnerable HTML conversion library. Third-party applications can also access the library via the WinWord Converter SDK. A variety of applications (Outlook, Outlook Express, Eudora, AOL, Lotus Notes, Adobe PhotoDeluxe, others) use the WebBrowser ActiveX control to interpret HTML documents.

By convincing a victim to view or convert a specially crafted HTML document (web page, HTML email message), an attacker could execute arbitrary code with the privileges of the victim. The attacker could also cause a denial of service.

Apply Patch
Apply the appropriate patch as referenced in Microsoft Security Bulletin MS03-023.

Disable Active scripting

Active scripting is required to automate the publicly announced attack. At a minimum, disable Active scripting in the Internet zone and the zone used by Outlook, Outlook Express, or any other application that uses Internet Explorer or the WebBrowser control to render HTML. Instructions for disabling Active scripting can be found in the CERT/CC Malicious Web Scripts FAQ. Disabling Active scripting is not a complete solution.

Apply the Outlook Email Security Update

Another way to effectively disable Active scripting in Outlook is to install the Outlook Email Security Update. The update configures Outlook to open email messages in the Restricted Sites Zone, where Active scripting is disabled by default. In addition, the update provides further protection against malicious code that attempts to propagate via Outlook. The Outlook Email Security Update is available for Outlook 98 and Outlook 2000. The functionality of the Outlook Email Security Update is included in Outlook 2002 and Outlook Express 6. Disabling Active scripting is not a complete solution.