Microsoft Security Bulletin (MS99-040)

Patch Available for "Download Behavior" Vulnerability
Originally Posted: September 28, 1999
Updated: October 08, 1999

Summary
On September 28, 1999, Microsoft released the original version of this bulletin, in order to  provide a workaround for a security vulnerability in Microsoft(r) Internet Explorer 5 that could  allow a malicious web site operator to read files on the computer of a person who visited the site. Microsoft has completed a patch that completely eliminates the vulnerability, and has  re-released this bulletin in order to advise customers of its availability.

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.

Affected Software Versions
Microsoft Internet Explorer 5

Patch Availability
The patch is available for download at either of the following
locations:
http://windowsupdate.microsoft.com
or
http://www.microsoft.com/msdownload/iebuild/dlbhav/en/dlbhav.htm

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, "Download Behavior" Vulnerability in Internet Explorer 5:
http://support.microsoft.com/support/kb/articles/Q242/5/42.asp.

(Note: It may take 24 hours from the original posting of this bulletin for this KB article to be visible.)
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.
October 08, 1999: Bulletin updated to announce availability of patch.

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