Microsoft Security Bulletin (MS00-043)

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Patch Available for "Malformed E-mail Header" Vulnerability

Originally posted: July 18, 2000

Summary

Microsoft has released a patch that eliminates a security
vulnerability in Microsoft(r) Outlook(r) and Outlook Express. Under
certain conditions, the vulnerability could allow a malicious user to
cause code of his choice to execute on another user's computer.

Frequently asked questions regarding this vulnerability and the
patch can be found at
http://www.microsoft.com/technet/security/bulletin/fq00-043.asp

Issue

A component shared by Outlook and Outlook Express contains an
unchecked buffer in the functionality that parses e-mail headers
when downloading mail via either POP3 or IMAP4. By sending an e-mail
that overruns the buffer, a malicious user could cause either of two
effects to occur when the mail was downloaded from the server by an
affected e-mail client:

• If the affected field were filled with random data, the
  e-mail could be made to crash.
• If the affected field were filled with carefully-crafted
  data, the e-mail client could be made to run code of the
  malicious user's choice.

Customers who have installed Internet Explorer 5.01 Service Pack 1,
and customers who have installed Internet Explorer 5.5 on any system
other than Windows 2000, would not be affected by this vulnerability.
Likewise, Outlook users who have configured Outlook to use only MAPI
services would not be affected, regardless of what version of
Internet Explorer they have installed.

Affected Software Versions

• Microsoft Outlook Express 4.0
• Microsoft Outlook Express 4.01
• Microsoft Outlook Express 5.0
• Microsoft Outlook Express 5.01
• Microsoft Outlook 97
• Microsoft Outlook 98
• Microsoft Outlook 2000

Patch Availability

The vulnerability can be eliminated by a default installation of
either of the following upgrades:

• Internet Explorer 5.01 Service Pack 1,
  http://www.microsoft.com/Windows/ie/download/ie501sp1.htm
• Internet Explorer 5.5 on any system except Windows 2000,
  http://www.microsoft.com/windows/ie/download/ie55.htm

Note: A non-default installation of IE 5.01 SP1 or IE 5.5 also will
eliminate this vulnerability, as long as an installation method is
chosen that installs upgraded Outlook Express components.

Note: When installed on a Windows 2000 machine, IE 5.5 does not
install upgraded Outlook Express components, and therefore does not
eliminate the vulnerability. However, Windows 2000 Service Pack 1
will install IE 5.5 and upgrade the Outlook Express components at
the same time.

Note: Patches will be available shortly that will eliminate the
vulnerability without requiring a full version upgrade. When they
are available, we will update this bulletin and re-release it.

Note: Additional security patches are available at the Microsoft
Download Center

More Information

Please see the following references for more information related to
this issue.

• Frequently Asked Questions: Microsoft Security Bulletin MS00-043,
  http://www.microsoft.com/technet/security/bulletin/fq00-043.asp
• Microsoft Knowledge Base article Q267884 discusses this issue
  and will be available soon.
• Microsoft TechNet Security web site,
  http://www.microsoft.com/technet/security/default.asp

Obtaining Support on this Issue

This is a fully supported patch. Information on contacting Microsoft
Technical Support is available at
http://support.microsoft.com/support/contact/default.asp.

Acknowledgments

Microsoft thanks USSR Labs (www.ussrback.com) for reporting this
issue to us and working with us to protect customers.

Revisions

• July 18, 2000: Bulletin Created.

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Last Updated July 18,2000

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