Microsoft Security Bulletin (MS00-045)
Patch Available for "Persistent Mail-Browser Link" Vulnerability
Originally Posted: July 20, 2000
Microsoft has released a patch that eliminates a security
vulnerability affecting Microsoft(r) Outlook Express. The
vulnerability could allow a malicious user to send an email that
would "read over the shoulder" of the recipient as he previews
subsequent emails in Outlook Express.
A patch is available that eliminates this vulnerability as well as
those discussed in Microsoft Security Bulletins MS00-043 and
MS00-046. Customers who already have taken the corrective action
discussed in either of these bulletins do not need to take any
Frequently asked questions regarding this vulnerability and the
patch can be found at
By design, HTML mail can contain script, and among the actions such a
script can take is to open a browser window that links back to the
Outlook Express windows. Also by design, script in the browser window
could read the HTML mail that is displayed in Outlook Express.
However, a vulnerability results because the link could be made
persistent. This could allow the browser window to retrieve the text
of mails subsequently displayed in the preview pane, and relay it to
the malicious user.
There are several significant restrictions on this vulnerability:
- Only the recipient could open the HTML mail that established
- The attack would only persist until the user either closed
the browser window that the HTML mail opened, or closed
- The malicious user could only read mails that were displayed
in the preview pane. If the preview pane feature were
disabled, he could not read mails under any conditions.
The vulnerability is eliminated in Outlook Express 5.5, and customers
who have installed it do not need to take any additional action.
Outlook Express 5.5 is available as part of Internet Explorer 5.01
Service Pack 1, and, except when installed on Windows 2000, Internet
Explorer 5.5. A patch is available for customers who prefer not to
upgrade to Outlook Express 5.5.
Affected Software Versions
- Microsoft Outlook Express 4.0
- Microsoft Outlook Express 4.01
- Microsoft Outlook Express 5.0
- Microsoft Outlook Express 5.01
This vulnerability can be eliminated by taking any of the following
- Installing the patch available at
- Performing a default installation of Internet Explorer 5.01
Service Pack 1,
- Performing a default installation of Internet Explorer 5.5
on any system except Windows 2000.
Note: The patch requires IE 4.01 SP2
IE 5.01 (http://www.microsoft.com/windows/ie/download/ie501.htm)
to install. Customers who install this patch on versions other
than these may receive a message reading "This update does not
need to be installed on this system". This message is incorrect.
More information is available in KB article Q261255.
Note: In addition to eliminating the vulnerability at issue
here, the steps above also eliminate all vulnerabilities
discussed in Microsoft Security Bulletins MS00-043
Customers who already have taken the corrective action discussed
in either of these bulletins do not need to take any additional
Note: Additional security patches are available at the Microsoft
Download Center (http://www.microsoft.com/downloads).
Please see the following references for more information related to
- Frequently Asked Questions: Microsoft Security Bulletin MS00-045,
- Microsoft Knowledge Base article Q261255 discusses this
vulnerability and will be available soon.
- Microsoft TechNet Security web site,
Obtaining Support on this Issue
This is a fully supported patch. Information on contacting Microsoft
Technical Support is available at
- June 20, 2000: Bulletin Created.
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Last Updated July 20, 2000