Microsoft Security Bulletin (MS99-043)

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Microsoft Security Bulletin (MS99-043)
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Workaround Available for "Javascript Redirect" Vulnerability
Originally Posted: October 18, 1999

Summary
=======
Microsoft has learned of a vulnerability in Microsoft(r) Internet Explorer
that could allow a  malicious web site operator to read files on the
computer of a user who visited the site, under  certain circumstances.
Microsoft is developing a patch that will eliminate the vulnerability; in
the meantime, a temporary workaround is discussed below.

Frequently asked questions regarding this vulnerability can be found
at http://www.microsoft.com/security/bulletins/MS99-043faq.asp.

Issue
=====
Client-local data that is displayed in the browser window can be made
available to the server by  using a redirect to a Javascript applet running
in the same window. This in effect bypasses  cross-domain security and makes
the data available to the applet, which could then send the data  to a
hostile server. This could allow a malicious web site operator to read the
contents of files  on visiting users' computers, if he or she knew the name
of the file and the folder in which it  resided. The vulnerability would not
allow the malicious user to list the contents of folders,  create, modify or
delete files, or to usurp any administrative control over the machine.

Affected Software Versions
==========================
 - Microsoft Internet Explorer 4.01 and 5.

Workaround
==========
As an interim step while the patch is under development, Microsoft
recommends that customers add  sites that they trust to the Trusted Zone,
and disable Active Scripting in the Internet Zone.  This will provide full
functionality for all trusted sites, while preventing untrusted sites from
being able to exploit this vulnerability. The FAQ provides details on how to
do this, and how to  manage Security Zones in general.

More Information
================
Please see the following references for more information related to this
issue.
 - Microsoft Security Bulletin MS99-043: Frequently Asked Questions,
   http://www.microsoft.com/security/bulletins/MS99-043faq.asp.
 - Microsoft Security Advisor web site,
   http://www.microsoft.com/security/default.asp.

Obtaining Support on this Issue
===============================
This is a fully supported workaround. Information on contacting Microsoft
Technical Support is available at
http://support.microsoft.com/support/contact/default.asp.

Revisions
=========
 - October 18, 1999: Bulletin Created.

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