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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. - -
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.
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