AdminSDHolder

AdminSDHolder is a special AD container with some "default" security permissions that is used as a template for protected AD accounts and groups (like Domain Admins, Enterprise Admins, etc.) to prevent their accidental and unintended modifications, and to keep them secure.

Once you have agained Domain Admin privileges, AdminSDHolder container can be abused by backdooring it by giving your user GenericAll privileges, which effectively makes that user a Domain Admin.

Execution

Backdooring the AdminSDHolder container by adding an ACL that provides user spotless with GenericAll rights for Domain Admins group:

Add-ObjectAcl -TargetADSprefix 'CN=AdminSDHolder,CN=System' -PrincipalSamAccountName spotless -Verbose -Rights All

This is actually what happens to the container - the security ACLs get updated and spotless gets all the privileges:

After 60+ minutes, the changes will propagate automatically, but if you want to force it (if you are testing this in your labs), you can do it via ldp.exe by Modifying DN like so:

Now, confirming that the user spotless has got GenericAll privileges against Domain Admins group:

Get-ObjectAcl -SamAccountName "Domain Admins" -ResolveGUIDs | ?{$_.IdentityReference -match 'spotless'}

We can now hop back to the Domain Admins any time we want to:

In fact, we do not even need to be part of DA group, we still have full access to the DC:

References