4ba7dfde5b
A null password allows logging into local PAM services such as "login" (agetty) and KDM. That's not actually a security problem for EC2 machines, since they do not have "local" logins; for VirtualBox machines, if you local access, you can do anything anyway. But it's better to be on the safe side and disable password-based logins for root.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.