Microsoft Security Bulletin MS99-051 - Important
Patch Available for "IE Task Scheduler" Vulnerability
Published: November 29, 1999
Originally Posted: November 29, 1999
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.
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
The vulnerability is eliminated by IE 5.01, which is available at:
- http://www.microsoft.com/downloads/search.aspx?displaylang=en/iebuild/ie501_win32/ en/ie501_win32.htm
Please see the following references for more information related to this issue.
- Microsoft Security Bulletin MS99-051: Frequently Asked Questions, http://www.microsoft.com/technet/security/bulletin/fq99-051.mspx.
Microsoft Knowledge Base (KB) article 246972, IE 5 Task Scheduler Allows Privilege Elevation on Windows NT Systems, http://support.microsoft.com/default.aspx?scid=kb;en-us;246972&sd=tech.
NOTE: It may take 24 hours from the original posting of this bulletin for this KB article to be visible.
Microsoft Security web site, http://www.microsoft.com/technet/security/default.mspx.
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.
Microsoft acknowledges Arne Vidstrom and Svante Sennmark for bringing this issue to our attention.
- November 29, 1999: Bulletin Created
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