Date: 11 March 2004
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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
AusCERT Security Bulletin Summary
Product: Microsoft Outlook 2002
Operating System: Windows XP
Impact: Execute Arbitrary Code/Commands
Access Required: Remote
CVE Names: CAN-2004-0121
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: --
* 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.
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.
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
Apply a patch
Apply the appropriate patch as specified by Microsoft Security
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
+ 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.
March 10, 2004: Initial release
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