I was installing Window Server 2012 Essentials today for a client, and had laid down the base operating system and then walked away from the computer for a while. When I came back, I found the screen was locked and I didn’t know the password. Some investigation found that the default Administrator password used during the setup of Windows Server 2012 Essentials is Admin@123 – Microsoft documented it here which is where I found it.
Note – this is not a major security issue, as this password is only used for the base Windows installation. Once you either finish the new server installation or server migration, the local administrator password is set to whatever you’ve told it to be set to for the Domain Administrator.