US-CERT Technical Cyber Security Alert TA04-070A -- Microsoft Outlook mailto URL Handling Vulnerability

From: CERT Advisory (
Date: 03/11/04

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    Date: Wed, 10 Mar 2004 21:10:02 -0500

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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)


       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

    III. Solution

    Apply a patch

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


       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

             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

              + 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.


         Please see Microsoft Security Bulletin MS04-009.

    Appendix B. References

         * US-CERT Vulnerability Note VU#305206 -
         * iDEFENSE Security Advisory 03.09.04 -
         * IETF RFC2368, "The mailto URL scheme" -
         * Microsoft Security Bulletin MS04-009 -
         * Microsoft Knowledge Base Article 307594 -
         * Microsoft Knowledge Base Article 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


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

       Copyright 2004 Carnegie Mellon University.

       Terms of use, see <>

       Revision History

       March 10, 2004: Initial release
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