When you add a new computer, it must first join the domain. If you use its future main user to do it, they’ll become the owner and be able to hijack the computer to become a local administrator in four easy steps.
During internal assessments in Active Directory environments, we often use BloodHound to find privilege escalation paths and we happen to find users with a
GenericAll relationship to their own workstations. This often means the IT joined these computers in a non-recommended way.
Joining Computers to a Domain
By default, any authenticated user can join up to 10 computers to the domain. The GPO configuration
Add workstation to domain defines who can join computers (
Authenticated Users by default), the maximum computer each user can add being defined by the property
10 by default).
When computers are joined to the domain that way, the account that joined it is granted several privileges on computer object (such as
In order not to give these privileges to normal users, it is recommended to use a dedicated account with the sole purpose of joining computers to the domain. It should have the
Create Computer objects right on the corresponding organizational unit (OU). When computers are joined by this account, different privileges are assigned on the computer object (
Owns in BloodHound) and the graph look like this:
In order to gain administrative privileges on the computer, the account that joined it to the domain has to use so-called Resource-Based Constrained Delegation. This technique has already been extensively described on the Internet, for additional details, see the references at the end of this post.
Basically, it consists of four steps:
- Gain access to an account – let’s call it
JOIN– who has
Ownsprivileges on the victim computer – called
- Gain access to a computer account – called
HELPER– e.g. by creating a new one with any
Authenticated Useror with
JOIN, or by becoming local administrator of another computer.
JOINto modify the
VICTIMand set it to trust
HELPER. This will allow
HELPERto impersonate domain users when handling with
HELPERto connect to
VICTIMand impersonate a user who can be delegated and who is a local administrator on
VICTIM, e.g. a domain administrator.
Remediations and Best Practices
In order to prevent this kind of exposure, you’ll have to prevent users from joining computers to the domain and use a dedicated account instead. First, remove the
Authenticated Users from the GPO configuration
Add workstations to domain (
Then, change the
ms-DS-MachineAccountQuota attribute from
Now, you can create a dedicated account with sole purpose and permission of joining computers to the domain by granting it the permission
Create Computer objects on the corresponding OU or by using the
Delegation of Control Wizard:
This account should have a strong password, that is changed regularly and that should only be known to administrators with regard to the permissions it confers.
Finally, you should protect every account with administrative privileges in general by setting the option
Account is sensitive and cannot be delegated. This can break things if you use (un-)constrained delegation with these accounts.
Fix What’s Already Done Wrong
Once this is in place, you might want to find the users with privileges on their computer due to previous practices and remove the permissions on the computer object.
To do so, you can use BloodHound to assess the situation. Once you have loaded the data from your domain in the application, mark the known and trusted join accounts as
High Value in order to exclude them from the analysis.
Then, run the following Raw Query to show all the ACLs from users to computers, which would result in a similar graph as the first screenshot in case computers were joined by regular user accounts:
MATCH p = (u:User)-[r]->(c:Computer) WHERE u.highvalue = false AND r.isacl = true RETURN p
Joining computers is not a task that you want to trust everyone with, since the joiner account can obtain administrative privileges on the joined computer. The recommended way to join computers is to have a dedicated account for this task.
In this post, we’ve seen the power of accounts joining computers to the domain and the importance to protect them. We’ve also observed that, as for many other points, it is important to harden the configuration of your Active Directory, since it is not shipped with a restrictive configuration by default.
Microsoft references about joining computers:
Several good explanations about delegation: