Microsoft Security Bulletin (MS99-040)

======================================



Workaround for IE 5 "Download Behavior"

Originally Posted: September 28, 1999



Summary

-------

Microsoft has learned of a vulnerability in Microsoft® Internet

Explorer 5 that could allow a malicious web site operator to take

inappropriate action on the computer of a person who visited the site.

Customers can immediately protect themselves against this vulnerability

by disabling Active Scripting in IE 5, as discussed in the FAQ.

Microsoft is also developing a patch that will restore safe operation

to the affected feature; when the patch is available, this bulletin

will be re-released.



Additional information and frequently asked questions regarding this

vulnerability can be found at

http://www.microsoft.com/security/bulletins/MS99-040faq.asp



Issue

-----

IE 5 includes a feature called "download behavior" that allows web page

authors to download files for use in client-side script.  By design, a

web site should only be able to download files that reside in its domain;

this prevents client-side code from exposing files on the user's machine

or local intranet to the web site.  However, a server-side redirect can be

used to bypass this restriction, thereby enabling a malicious web site

operator to read files on the user's machine or the user's local intranet.



This vulnerability would chiefly affect workstations that are connected to

the Internet. As an immediate measure, customers can prevent the download

behavior function from operating by disabling Active Scripting, as discussed

in the FAQ.  A patch that restores correct operation is under development

and will be delivered shortly.



Affected Software Versions

==========================

 - Microsoft Internet Explorer 5



Workaround

==========



The vulnerability can be prevented by disabling Active Scripting.  The FAQ

contains details on how to do this.



More Information

================

Please see the following references for more information related to this

issue.

 - Microsoft Security Bulletin MS99-040: Frequently Asked Questions,

   http://www.microsoft.com/security/bulletins/ms99-040faq.asp.

 - Microsoft Knowledge Base (KB) article Q242542 Workaround available for

   download behavior vulnerability,

   http://support.microsoft.com/support/kb/articles/Q242/5/42.asp.

   (Note: It may take 24 hours from the posting of this bulletin for the

   KB article to appear)

 - Microsoft Security Advisor web site,

   http://www.microsoft.com/security/default.asp.



Obtaining Support on this Issue

===============================

Information on contacting Microsoft Technical Support is available

at http://support.microsoft.com/support/contact/default.asp.



Acknowledgments

===============

Microsoft acknowledges Georgi Guninski for bringing this issue to our

attention.



Revisions

=========

 - September 28, 1999: Bulletin Created.



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