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             AUSCERT External Security Bulletin Redistribution

     ESB-2004.0202 -- US-CERT Technical Cyber Security Alert TA04-070A
            Microsoft Outlook mailto URL Handling Vulnerability
                               11 March 2004

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        AusCERT Security Bulletin Summary
        ---------------------------------

Product:                Microsoft Outlook 2002
Publisher:              US-CERT
Operating System:       Windows XP
                        Windows 2000
                        Windows NT
                        Windows ME
                        Windows 98SE
                        Windows 98
Impact:                 Execute Arbitrary Code/Commands
Access Required:        Remote
CVE Names:              CAN-2004-0121

Ref:                    ESB-2004.0191

Comment: Microsoft has updated the severity of this vulnerability from
         "Important" to "Critical". The reason for this change is
         explained on the Microsoft site:

         '... it was determined that this vulnerability could also affect
         users who do not have the "Outlook Today" folder home page as
         their default home page in Outlook 2002.'

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Microsoft Outlook mailto URL Handling Vulnerability

   Original issue date: March 10, 2004
   Last revised: --
   Source: US-CERT

Systems Affected

     * Microsoft Office XP (up to Service Pack 2)
     * Microsoft Outlook 2002 (up to Service Pack 2)

Overview

   A vulnerability in the way that Microsoft Outlook 2002 handles a
   certain type of URL could allow a remote attacker to execute arbitrary
   code on the vulnerable system.

I. Description

   Microsoft Outlook provides a centralized application for managing and
   organizing email messages, schedules, tasks, notes, contacts, and
   other information. Outlook is included as a component of newer
   versions of Microsoft Office and available as a stand-alone product.

   Outlook 2002 exposes a vulnerability due to inadequate checking of
   parameters passed to the Outlook email client. The vulnerability is
   caused by the way a "mailto:" URL is interpreted. An attacker creating
   specially formatted "mailto:" URLs can cause Outlook to run privileged
   script, ultimately leading to the execution of arbitrary code. The
   malicious code could be delivered to the victim via a specially
   crafted HTML email message or from an intruder-controlled web page.

   Microsoft originally stated that users were only at risk from this
   vulnerability when Outlook 2002 is configured as the default mail
   reader and when the "Outlook Today" home page is their default folder
   home page. Subsequent information has been published that indicates
   that this is not true and users in other situations are vulnerable via
   a slightly different attack vector.

II. Impact

   An attacker could execute arbitrary code of their choosing on the
   system running the vulnerable version of Outlook. Upon successful
   exploitation, the malicious code would be executed in the context of
   the "Local Machine" Internet Explorer zone under the user running
   Outlook.

III. Solution

Apply a patch

   Apply the appropriate patch as specified by Microsoft Security
   Bulletin MS04-009.

Workarounds

   Microsoft recommends the following workarounds for users who are
   unable to apply the patches:

     * Do not use the "Outlook Today" folder home page in Outlook 2002
       You can help protect against this vulnerability by turning off the
       "Outlook today" folder home page in Outlook 2002.

         1. In the "Folder List" window of Outlook, right-click on
            "Outlook Today" or "Mailbox - [User Name]"

         2. Select Properties for "Outlook Today" or "Mailbox - [User
            Name]"

         3. Select "Home Page" tab

         4. Uncheck "Show home page by default for this folder"

         5. Repeat for all other "Folder List" items labeled "Outlook
            Today" or "Mailbox - [User Name]"

       Impact of Workaround: The "Outlook Today" folder home page would
       no longer be available.

     * If you are using Outlook 2002 or Outlook Express 6.0 SP1 or later,
       read email messages in plain text format to help protect yourself
       from the HTML email attack vector

       Microsoft Outlook 2002 users who have applied Service Pack 1 or
       later and Outlook Express 6.0 users who have applied Service Pack
       1 or later can enable a feature that will enable them to view all
       non-digitally-signed email messages or non-encrypted email
       messages in plain text only. Digitally-signed email messages and
       encrypted email messages are not affected by the setting and may
       be read in their original formats.

       Instructions for enabling these settings can be found at the
       following locations:

          + Outlook 2002 - Microsoft Knowledge Base Article 307594

          + Outlook Express 6.0 - Microsoft Knowledge Base Article 291387

       Impact of Workaround: Email that is viewed in plain text format
       cannot contain pictures, specialized fonts, animations, or other
       rich content. Additionally:

          + The changes are applied to the preview pane and to open
            messages.

          + Pictures become attachments to avoid loss of message content.

          + The object model (custom code solutions) may behave
            unexpectedly because the message is still in Rich Text Format
            or in HTML format in the mail store.

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 MS04-009.

Appendix B. References

     * US-CERT Vulnerability Note VU#305206 -
       <http://www.kb.cert.org/vuls/id/305206>
     * iDEFENSE Security Advisory 03.09.04 -
       <http://www.idefense.com/application/poi/display?id=79&type=vulner
       abilities>
     * IETF RFC2368, "The mailto URL scheme" -
       <http://www.ietf.org/rfc/rfc2368.txt>
     * Microsoft Security Bulletin MS04-009 -
       <http://microsoft.com/technet/security/bulletin/MS04-009.aspx>
     * Microsoft Knowledge Base Article 307594 -
       <http://support.microsoft.com/default.aspx?kbid=307594>
     * Microsoft Knowledge Base Article 291387 -
       <http://support.microsoft.com/default.aspx?kbid=291387>
     _________________________________________________________________

   This issue was jointly reported publicly by Microsoft Security and
   iDefense. They, in turn, credit Juoko Pyonen with the discovery and
   research of this vulnerability. Information from iDefense and
   Microsoft was used in this document.
     _________________________________________________________________

   This document is also available online at

   <http://www.us-cert.gov/cas/techalerts/TA04-070A.html>

   Feedback can be directed to the authors, Chad Dougherty and Jeff
   Havrilla.
     _________________________________________________________________

   Copyright 2004 Carnegie Mellon University.

   Terms of use, see <http://www.us-cert.gov/legal.html>

   Revision History

   March 10, 2004: Initial release
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