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  • CERT Advisory CA-2003-14 Buffer Overflow in Microsoft Windows HTML Conversion Library

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Original issue date: July 14, 2003
Last revised: --
Source: CERT/CC

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

Systems Affected

  • Windows 98 and 98 Second Edition (SE)
  • Windows NT 4.0 and 4.0 Terminal Server Edition (TSE)
  • Windows Millennium Edition (Me)
  • Windows 2000
  • Windows XP
  • Windows Server 2003


A buffer overflow vulnerability exists in a shared HTML conversion library included in Microsoft Windows. An attacker could exploit this vulnerability to execute arbitrary code or cause a denial of service.

I. Description

Microsoft Windows includes a shared HTML conversion library (html32.cnv). According to Microsoft Security Bulletin 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 vulnerability that can be triggered by a specially crafted align attribute in an <HR> element. The library can be loaded by any application on the system. For example, Internet Explorer (IE) uses the library to handle HTML data stored in the clipboard. Using script, an attacker can cause IE to copy a crafted <HR> element into the clipboard and load the library. The attacker could accomplish this by convincing a victim to view an HTML web page or HTML email message with IE, Outlook, or Outlook Express in a zone where Active scripting and Allow paste operations via script are enabled.

This vulnerability is not limited to IE, Outlook, or Outlook Express. Any program, including non-Microsoft applications, can use the vulnerable library and may present other vectors of attack.

Further information is available in VU#823260. Common Vulnerabilities and Exposures (CVE) refers to this issue as CAN-2003-0469.

II. Impact

An attacker could execute arbitrary code with the privileges of the process that loaded the HTML conversion library. The attacker could also crash the process, causing a denial of service.

III. Solution

Apply a patch

Apply the appropriate patch as specified by Microsoft Security Bulletin MS03-023.

Modify Internet Explorer security zone configuration

Modify one or both of the following IE security zone settings in the Internet zone and the zone(s) used by Outlook, Outlook Express, and any other application that uses Internet Explorer or the WebBrowser ActiveX control to render HTML:

  • Set Allow paste operations via script to Disable

  • Set Active scripting to Disable
Either of these changes will prevent attacks that depend on scripting in the IE HTML rendering engine. However, these changes are not complete solutions, and they do not prevent attacks that use other vectors.

Note that disabling Active scripting provides defense against other attacks that are outside the scope of this document.

Instructions for modifying IE 5 security zone settings can be found in the CERT/CC Malicious Web Scripts FAQ. In IE 6, the High security zone setting includes both of these changes.

Appendix A. Vendor Information

This appendix contains information provided by vendors. When vendors report new information, this section is updated and the changes are noted in the revision history. If a vendor is not listed below, we have not received their comments.


Please see Microsoft Security Bulletin MS03-023.

Appendix B. References

This vulnerability was publicly reported by Digital Scream.

Feedback can be directed to the author, Art Manion.

Copyright 2003 Carnegie Mellon University.

Revision History

July 14, 2003: Initial release

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