ISS Security Alert
October 21, 1997
Scheduler/Winlogin Keys have Incorrect Permissions
This advisory describes two similar configuration problems in the Windows
NT Registry key permissions. These vulnerabilities can allow users with
Server Operator privilege to increase their access level to Administrator.
Problem 1: Scheduler Key Has Incorrect Permissions
Affects: Windows NT
Description:
The HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Schedule key
controls the schedule service. Server Operators have permission to write
to this registry tree, which would allow them to manually schedule jobs
to be run by the schedule service, which normally executes under the
system user context. This can be used to raise the Server Operator's
access level to Administrator.
Risk: Medium
Solution:
Local Machine (GUI): From the Start menu, choose 'Run.' Type 'regedt32'
and click 'OK.' This opens the Registry Editor. Through the Security
menu, remove write access to the Schedule key for Server Operators.
Problem 2: Winlogon Key Has Incorrect Permissions
Affects: Windows NT
Description:
The HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\Winlogon
key has two values which can be used to cause a process to execute upon
either system bootup, or when a user logs on. The programs pointed to by
the System value run under the system user context after boot, and could
be used to change a user's rights or access level. The UserInit value
runs applications when a user logs in. The default settings for this key
allow Server Operators to write these values, either of which could be
used to raise a System Operator's access level to Administrator.
Risk: Medium
Solution:
Local Machine (GUI): From the Start menu, choose 'Run.' Type 'regedt32'
and click 'OK.' This opens the Registry Editor. Through the Security
menu, remove write access to the Winlogon key for Server Operators.
=========================================================================
Caution: Care must be taken when using the Registry Editor. If incorrect
values are entered, the system may become inoperable. Should a mistake be
made when editing the registry values, the registry state can be restored
to the state at the last time the system booted up. For more information,
see the Windows NT Help under the "Registry" section.
=========================================================================
Acknowledgments:
This problem was identified by David LeBlanc of ISS (dleblanc@iss.net).
References:
http://support.microsoft.com/support/kb/articles/q126/7/13.asp
http://www.infoworld.com/cgi-bin/displayStory.pl?971014.wntsecurity.htm
X-Force Vulnerability and Threat Database: http://www.iss.net/xforce
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Copyright (c) 1997 by Internet Security Systems, Inc.
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