Nothing will prevent that. The question is, what would you do with the stolen credentials in terms of privilege escalation? Suppose we are in a situation where

Nothing will prevent that. The question is, what would you do with the stolen credentials in terms of privilege escalation? Suppose we are in a situation where the UAC is on and asks for every action on the secure desktop. Even if you were to authenticate with the stolen credentials, you would end up in an unprivileged session due to UAC's token filtering mechanisms. If you were then to request elevation, the UAC would prompt the user on the secure desktop, which you cannot control from within the user session. It is therefore not enough to imitate the prompt. You also need to convince the user to accept the real prompt.