As per samba upstream advisory:

Tickets received by the kpasswd service were decrypted without specifying that only that service's own keys should be tried. By setting the ticket's server name to a principal associated with their own account, or by exploiting a fallback where known keys would be tried until a suitable one was found, an attacker could have the server accept tickets encrypted with any key, including their own.

A user could thus change the password of the Administrator account and gain total control over the domain. Full loss of confidentiality and integrity would be possible, as well as of availability by denying users access to their accounts.

In addition, the kpasswd service would accept tickets encrypted by the krbtgt key of an RODC, in spite of the fact that RODCs should not have been able to authorise password changes.