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Microsoft Security Bulletin MS99-051 - Important

Patch Available for "IE Task Scheduler" Vulnerability

Published: November 29, 1999

Version: 1.0

Originally Posted: November 29, 1999

Summary

Microsoft has released a version upgrade that eliminates a vulnerability in Microsoft® 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/technet/security/bulletin/fq99-051.mspx.

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.

Vulnerability Identifier: CVE-1999-0839

Patch Availability

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

More Information

Please see the following references for more information related to this issue.

Obtaining Support on this Issue

This is a fully supported patch. Information on contacting Microsoft Technical Support is available at http://support.microsoft.com/contactussupport/?ws=support.

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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