AdminSDHolder
Theory
AdminSdHolder protects domain objects against permission changes. "AdminSdHolder" either refers to a domain object, a "worker code" or an operation depending on the context.
The operation consists in the PDC (Principal Domain Controller) Emulator restoring pre-set permissions for high-privilege users every 60 minutes. Understanding what DACLs/ACEs are and how to abuse them is a requirement to the understanding of this persistence technique (see Access Controls abuse).
The operation is conducted by a "worker code" called SDProp (Security Descriptor propagator).
SDProp propagates AdminSdHolder's Security Descriptor (which contains the DACL) to every protected object every 60 minutes if their SD is different.
The AdminSdHolder object is located at CN=AdminSdHolder,CN=SYSTEM,DC=DOMAIN,DC=LOCAL. For instance, the default AdminSdHolder object's DACL contains the following.
- Authenticated Users: Read
- SYSTEM: Full Control
- Administrators: Modify
- Domain Admins: ReadAndExecute
- Enterprise Admins: ReadAndExecute
The default protected objects are the following.
- member users (possibly nested) of the following groups:
Account Operators,Administrators,Backup Operators,Domain Admins,Domain Controllers,Enterprise Admins,Print Operators,Read-only Domain Controllers,Replicator,Schema Admins,Server Operators - the following users:
Administrator,krbtgt
It's worth noting the members of the Domain Controllers and Read-Only Domain Controllers groups are not protected (source), only the groups are.
When talking about AdminSdHolder, the AdminCount attribute is usually mentioned. This attribute is automatically set on an object when adding it to a protected group. Originally, the purpose was to improve SDProp's performance. AdminCount cannot be used for malicious purposes and is now mainly informative.
Also, AdminCount will block inheritance. This means that setting inheritence in the ACE added to AdminSdHolder will be mostly useless. For instance, domain controllers objects are part of the Domain Controller group (at CN=Domain Controllers,CN=Users) which has AdminCount set to 1. While a new ACE set to AdminSdHolder will propagate to the group, its members will not inherit it.
Practice
Once sufficient privileges are obtained, attackers can abuse AdminSdHolder to get persistence on the domain by modifying the AdminSdHolder object's DACL.
Let's say an attacker adds the following ACE to AdminSdHolder's DACL: attackercontrolleduser: Full Control.
At the next run of SDProp, attackercontrolleduser will have a GenericAll privilege over all protected objects (Domain Admins, Account Operators, and so on).
From UNIX-like systems, this can be done with Impacket's dacledit.py (Python).
dacledit.py -action 'write' -rights 'FullControl' -principal 'controlled_object' -target-dn 'CN=AdminSDHolder,CN=System,DC=DOMAIN,DC=LOCAL' 'domain'/'user':'password'AdminSdHolder's DACL can then be inspected with the same utility.
dacledit.py -action 'read' -target-dn 'CN=AdminSDHolder,CN=System,DC=DOMAIN,DC=LOCAL' 'domain'/'user':'password'