Group Policy can map to Sites, Domain and OUs. If group policy is mapped to OU, by default it will apply to any object under it. But within a OU, Domain or Site there are lots of objects. The security, system or application settings requirements covers by group policies not always applies to boarder target groups. Group Policy filtering capabilities allows to further narrow down the group policy target to security groups or individual objects. 

There are few different ways we can do the filtering in group policy.

1) Security Filtering

2) WMI Filtering

In this post we are going to look in to Security Filtering. In one of my previous post I already covered WMI filtering. It can be found under http://www.rebeladmin.com/2018/02/group-policy-wmi-filters-nutshell/ 

Before apply the security filtering, the first thing to make sure is group policy mapped correctly to the Site, Domain or OU. The security group or the objects you going to target should be under correct level where group policy is mapped. 

We can use the GMPC or PowerShell cmdlets to add the security filtering to GPO.

gsec1
As you can see, by default any policy have “Authenticated Users” group added to the security filtering. It means by default the policy will apply to any authenticated user in that OU. When we add any group or object to security filtering, it also creates entry under delegation. In order to apply a group policy to an object, it needs minimum of,
 
1) READ
2) APPLY GROUP POLICY
 
Any object added to the Security Filtering section will have both of these permissions set by default. Same way if an object added directly to delegation section and apply both permissions, it will list down those objects under Security Filtering section. 
Now, before we add custom objects to the filtering, we need change the default behavior of the security filtering with “Authenticated Users”. Otherwise it doesn’t matter what security group or object you add it will still apply group policy settings to any authenticated user. Before Microsoft release security patch MS16-072 in year 2016, we can simply remove the Authenticated Users group and add the required objects to it. with this new security patch changes, group policies now will run with in computer security context. Before it was executed with in user’s security context. In order to accommodate this new security requirements, one of following permissions must be available under group policy delegation. 
 
Authenticated Users – READ
Domain Computers – READ
 
In order to edit these changes, Go to Group Policy, Then to Delegation tab, Click on Advanced, Select Authenticated users and then remove Apply group policy permissions. 
 
gsec2
 
Now we can go back to Scope tab and add the required security group or objects in to security filtering section. it will automatically add the relevant Read and Apply Group Policy permissions. 
 
gsec3
 
In here we looking in to how to apply group policy to specific target, but it also allows to explicitly allow it to large number of objects and block groups or object by applying it. as an example, let’s assume we have a OU with few hundred objects from different classes. From all these we have like 10 computer objects which we do not need to apply a given group policy. Which one is easy? go and add each and every security group and object to security filtering or allow every one for group policy and block it only for one security group? Microsoft allows to use the second method in filtering too. In order to do that, group policy should have default security filtering which is “Authenticated users” with READ and APPLY GROUP POLICY permissions. Then go to Delegation tab and click on Advanced option. In next window click on Add button and select the group or object that you need to block access to. 
 
gsec4
 
Now in here we are denying READ and APPLY GROUP POLICY permissions to an object. So, it will not able to apply the group policy and all other object under that OU will still able to read and apply group policy. Easy ha?
 
This marks the end of this blog post. If you have any questions feel free to contact me on rebeladm@live.com also follow me on twitter @rebeladm to get updates about new blog posts.
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