In November 2021, two vulnerabilities caught the attention of many security researchers as they could allow domain escalation from a standard user.
CVE-2021-42278 - Name impersonation
Computer accounts should have a trailing $ in their name (i.e. sAMAccountName attribute) but no validation process existed to make sure of it. Abused in combination with CVE-2021-42287, it allowed attackers to impersonate domain controller accounts.
CVE-2021-42287 - KDC bamboozling
When requesting a Service Ticket, presenting a TGT is required first. When the service ticket is asked for is not found by the KDC, the KDC automatically searches again with a trailing $. What happens is that if a TGT is obtained for bob, and the bob user gets removed, using that TGT to request a service ticket for another user to himself (S4U2self) will result in the KDC looking for bob$ in AD. If the domain controller account bob$ exists, then bob (the user) just obtained a service ticket for bob$ (the domain controller account) as any other user 🤯.
Machine Account
The ability to edit a machine account's sAMAccountName and servicePrincipalName attributes is a requirement to the attack chain. The easiest way this can be achieved is by creating a computer account (e.g. by leveraging the MachineAccountQuota domain-level attribute if it's greater than 0). The creator of the new machine account has enough privileges to edit its attributes. Alternatively, taking control over the owner/creator of a computer account should do the job.
The attack can then be conducted as follows.
Clear the controlled machine account servicePrincipalName attribute of any value that points to its name (e.g. host/machine.domain.local, RestrictedKrbHost/machine.domain.local)
Change the controlled machine account sAMAccountName to a Domain Controller's name without the trailing $ -> CVE-2021-42278
Request a TGT for the controlled machine account
Reset the controlled machine account sAMAccountName to its old value (or anything else different than the Domain Controller's name without the trailing $)
Request a service ticket with S4U2self by presenting the TGT obtained before -> CVE-2021-42287
Get access to the domain controller (i.e. DCSync)
See if we have access to create a new computer
Download noPAC.py
Scanning to see if we have access
Dumping Dumping Dumping Love love
Exploiting and getting Shell
cleanup cleanup, everybody lets cleanup