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Title:    Q318593: Opening Group Policy Files for Exclusive Read 
                Blocks Policy Application
Date:    04 April 2002
Software:
Microsoft Windows 2000 Server 
Microsoft Windows 2000 Advanced Server 
Microsoft Windows 2000 Datacenter Server
Impact:    Attacker could block application of Group Policy.
Recommendation:  Administrators should consider applying the patch to
domain controllers.
Max Risk:  Moderate
Bulletin:  MS02-016


Microsoft encourages customers to review the Security Bulletin at: 
http://www.microsoft.com/technet/security/bulletin/MS02-016.asp.
- -
- ----------------------------------------------------------------------

Issue:
======
Group Policy in Windows 2000 is implemented by storing data in the
Active Directory and the system volume on the domain controller. This
storage location is called the Group Policy Object (GPO). When a
machine or user logs onto the domain, it reads the GPO and applies
the settings it contains. Most of these settings are also refreshed
by default every 90 minutes. However, like most operating systems,
Windows 2000 provides several types of read access, including
exclusive-read, and this could enable an attacker to lock the Group
Policy files, thereby allowing a user to prevent Group Policy from
being applied for all users affected by the GPO. 

An attacker would likely exploit the vulnerability by first logging
onto the domain normally, and then opening the Group Policy files
with exclusive read access. She could then log onto the network a
second time. Because the policy files would be locked, the second
logon would occur without Group Policy being applied. The result
would be that, although all previous Group Policy settings on the
second machine would remain in force, any new policy settings would
not be applied. The attacker's second session would take place using
what policy settings had most recently been applied. 

The effect wouldn't be limited only to the attacker. Any other user
who happened to log onto the network while the Group Policy files
were locked would also do so without new policy settings being
applied. However, users who weren't involved in the attack might be
unable to determine that policy had been blocked. Group Policy
application is a transparent process, so such a user would likely be
unaware that the intended policy settings have not been applied. 

Mitigating Factors:
===================
- The vulnerability would enable an attacker to block the application
of new Group Policy settings, but any settings that had been applied
during previous logons would remain in force. 
- The vulnerability could only be exploited by a user who had a bona
fide userid and password on the network. 
- The specific gain for the attacker would depend on the extent to
which the administrator had customized Group Policy on the domain. 
- The vulnerability would provide no way for an attacker to change
Group Policy, or to gain user group memberships. 
- An administrator could determine the attacker's identity by using
the Shared Folders MMC snap-in to view the userid of the person who
had the policy files open. 

Risk Rating:
============
- - Internet Systems:  Low
- - Intranet Systems:  Moderate
- - Client Systems:  None

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms02-016.asp
   for information on obtaining this patch.

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