Alert: Microsoft Security Bulletin - MS02-016

From: Russ (Russ.Cooper@RC.ON.CA)
Date: 04/05/02

Date:         Thu, 4 Apr 2002 18:25:13 -0500
From: Russ <Russ.Cooper@RC.ON.CA>

Q318593: Opening Group Policy Files for Exclusive Read Blocks Policy Application

Originally posted: April 04, 2002


Who should read this bulletin: Network administrators using Microsoft® Windows® 2000 domain controllers.

Impact of vulnerability: Attacker could block application of Group Policy.

Maximum Severity Rating: Moderate

Recommendation: Administrators should consider applying the patch to domain controllers.

Affected Software:
- Microsoft Windows 2000 Server
- Microsoft Windows 2000 Advanced Server
- Microsoft Windows 2000 Datacenter Server

Technical description:

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.

Vulnerability identifier: CAN-2002-0051

This email is sent to NTBugtraq automatically as a service to my subscribers. Since its programmatically created, and since its been a long time since anyone paid actual money for my programming skills, it may or may not look that good...;-]

I can only hope that the information it does contain can be read well enough to serve its purpose.

Russ - Surgeon General of TruSecure Corporation/NTBugtraq Editor