Tips and Tricks
An Easy Fix for a Sticky GPO Security Problem
Ease the pain by automating account creation.
- By Derek Melber
Group Policy Objects (GPO) provide a powerful way to ensure that
users adhere to corporate computing policies. But many organizations
may be subject to a security vulnerability stemming from GPO settings
used for servers and client computers in Active Directory domains.
Fortunately, this security problem has an easy fix.
The problem stems from the Registry changes that occur when you
configure a policy under Computer Configuration|Windows Settings|Security
Settings|Local Policies|Security Options. Most of these settings
are simple Registry value changes that affect the computer the GPO
targets. That means any user with administrative access to the Registry
on the target computer can change the setting, even if the GPO policy
configures the setting.
This might come as something of a shock, since Microsoft likes
to claim that GPOs are bulletproof, secure, non-tattooing and stable.
I won't try to make the case that such claims are 100 percent wrong,
but in this case, at least, some education and guidance is in order.
Let's walk through a real-world example to help visualize the
issue. Assume you've enabled the "Do not display last user
name in logon screen" policy in a GPO that affects all computers
and users in the domain, including administrators. When the logon
screen appears following the CTRL-ALT-DEL key sequence, this policy
removes the username of the last person who logged on to the computer,
increasing security by obfuscation. You apply the policy to every
user who logs on to the computer, including administrators.
Now the rub: Assume that Joe is the user who uses the computer
account named Joe_XP1. Company policy requires users to be local
administrators on their own computers so they can install applications
and security updates. Therefore, Joe has administrative privileges
on Joe_XP1. With this access, Joe can open the Registry editor,
find the DontDisplayLastUserName Registry value, and change the
value to 0—thus disabling the policy. The next time Joe (or
any user) logs on to Joe_XP1, the username will appear in the Username
textbox on the logon screen.
What about after the GPO has time to refresh or the computer is
restarted? Neither action will fix the problem, because the GPO
won't think anything's wrong. The GPO is aware only of the GPO version
number, not the actual policy settings of that version. This version
number is stored in the GPO and on the computer that it updates.
In our example, the Registry value has changed, but not the GPO
version number. Since the two version numbers match, the GPO passes
on any configurations for the computer.
The solution to this problem is quite simple. A GPO policy, named
"Security policy processing," controls how to handle GPO
refreshes with regard to version checking. The policy is located
under the Computer Configuration|Administrative Templates|System|Group
Policy node in the GPO. When configuring this policy you'll see
a check box labeled "Process even if the Group Policy objects
have not changed." When this is checked, the GPO version is
not evaluated for the Security Options settings in the GPO. Instead,
all of the settings are configured on the computer as if it were
the first time the GPO was being applied. This will occur at every
refresh interval (by default every 90 minutes) and every time the
computer is restarted.
Other settings to consider include:
- Use GPOs to deploy software, eliminating the need for users
to be local administrators.
- Use GPOs to disable users from using Registry editing tools.
Even though the default GPOs don't combat local changes to GPO
settings, there are GPO settings that can ensure the security is
enforced on target computers.
Derek Melber (MCSE, MVP, CISM) is president of BrainCore.Net AZ, Inc., as well as an independent consultant and speaker, as well as author of many IT books. Derek educates and evangelizes Microsoft technology, focusing on Active Directory, Group Policy, security and desktop management. As one of only 8 MVPs in the world on Group Policy, Derek’s company is often called upon to develop end-to-end solutions regarding Group Policy for companies. Derek is the author of the The Group Policy Resource Kit by MSPress, which is the defacto book on the subject.