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

   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


Overview

   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.

Microsoft

     Please see Microsoft Security Bulletin MS03-023.


Appendix B. References

     * CERT/CC Vulnerability Note VU#823260 -
       <http://www.kb.cert.org/vuls/id/823260>
     * Microsoft Security Bulletin MS03-023 -
       <http://microsoft.com/technet/security/bulletin/MS03-023.asp>

     _________________________________________________________________

   This vulnerability was publicly reported by Digital Scream.
     _________________________________________________________________

   Feedback can be directed to the author, Art Manion.
   ______________________________________________________________________

   This document is available from:
   <http://www.cert.org/advisories/CA-2003-14.html>
   ______________________________________________________________________

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   Revision History

   July 14, 2003: Initial release