Vulnerability Note VU#305206

Microsoft Outlook fails to properly filter parameters passed via "mailto:" URL

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. By creating a specially formatted mailto: URL an attacker may be able to alter the way that Outlook is invoked in a manner that allows code execution. The malicious code could be delivered to the victim via a specially-crafted HTML email message or from an intruder-controlled webpage.

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

Microsoft has published Microsoft Security Bulletin MS04-009 in response to this issue. Users are encouraged to review this bulletin and apply the patches it refers to.

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 e-mail messages in plain text format to help protect yourself from the HTML e-mail 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 e-mail messages or non-encrypted e-mail messages in plain text only.

  Digitally-signed e-mail messages and encrypted e-mail messages are not affected by the setting and may be read in their original formats.

  See Microsoft Knowledge Base Article 307594 for information about how to enable this setting in Outlook 2002.

  See Microsoft Knowledge Base Article 291387 for information about how to enable this setting in Outlook Express 6.0

  Impact of Workaround: E-mail 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.
  • Because the message is still in Rich Text Format or in HTML format in the store, 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.

http://www.microsoft.com/technet/security/bulletin/ms04-009.mspx
http://www.idefense.com/application/poi/display?id=79&type=vulnerabilities
http://www.ietf.org/rfc/rfc2368.txt
http://support.microsoft.com/default.aspx?kbid=307594
http://support.microsoft.com/default.aspx?kbid=291387

Credit

This issue was jointly reported publicly by Microsoft Security and iDefense. They, in turn, credit Jouko Pynnönen with the discovery and research of this vulnerability.

This document was written by Chad R Dougherty based on information provided by Microsoft and iDefense.

Other Information

If you have feedback, comments, or additional information about this vulnerability, please send us email.