Microsoft Security Bulletin (MS99-051)

--------------------------------------



Patch Available for "IE Task Scheduler" Vulnerability

Originally Posted: November 29, 1999



Summary

=======

Microsoft has released a version upgrade that eliminates a vulnerability in

Microsoft(r) Internet Explorer 5. A vulnerability in an optional component

could allow a malicious user to gain additional privileges on a Windows NT

machine that allowed him or her to create or change files.



Frequently asked questions regarding this vulnerability can be found at

http://www.microsoft.com/security/bulletins/MS99-051faq.asp.



Issue

=====

IE 5 includes an Offline Browsing Pack that is not installed by default. The

Offline Browsing Pack provides a Task Scheduler that replaces the native

Windows NT Schedule Service (the schedule service is also known as the "AT

Service"). A vulnerability in the Task Scheduler poses a privilege elevation

risk and could allow normal users to execute code on the local machine in

System context. (The Windows NT Schedule Service does not have this

vulnerability).



The IE 5 Task Scheduler controls who can create and submit "AT jobs." The

utility that is used to create AT jobs can only be run by an administrator,

and the Task Scheduler will only execute AT jobs that are owned by

administrators. However, if a malicious user had change access to an

existing file owned by an administrator (it would not need to be an AT job),

he or she could modify it to be a valid AT job and place in the appropriate

folder for execution. This would bypass the control mechanism and allow the

job to be executed.



This vulnerability would primarily affect machines that allow normal users

to interactively log onto them. The patch eliminates this vulnerability by

digitally signing all AT jobs at creation time, and verifying the signature

at execution time.



Affected Software Versions

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

 - Microsoft Internet Explorer 5, when run on a Windows NT 4.0 system



NOTE: The affected components are part of the IE 5 Offline Browsing Pack,

which is not installed by default.



NOTE: Windows NT 4.0 includes a native scheduling service, but it does not

have this vulnerability.



Patch Availability

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

The vulnerability is eliminated by IE 5.01, which is available at:

 - http://www.microsoft.com/msdownload/iebuild/ie501_win32/

   en/ie501_win32.htm



NOTE: A line break has been inserted into the above URL for readability.



More Information

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

Please see the following references for more information related to this

issue.

 - Microsoft Security Bulletin MS99-051: Frequently Asked Questions,

   http://www.microsoft.com/security/bulletins/MS99-051faq.asp.

 - Microsoft Knowledge Base (KB) article Q246972,

   IE 5 Task Scheduler Allows Privilege Elevation on Windows NT Systems,

   http://support.microsoft.com/support/kb/articles/q245/7/29.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

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

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 acknowledges Arne Vidstrom and Svante Sennmark for bringing this

issue to our attention.



Revisions

=========

 - November 29, 1999: Bulletin Created



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