The MainWP Child WordPress plugin before 6.1.2 does not verify the requesters identity in its site-registration request handler when password authentication has been disabled for the targeted account, allowing an unauthenticated attacker to obtain a valid authentication session as that account, including an administrator, by naming its login in a single registration request.
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.