Major linux bug allows root shell by holding enter key
zdnet.com
zdnet.com
When you have a physical access to the hard disk, you can do the same things without exploiting this bug.
Now, you should have this sort of access seriously locked down, since hardware vendors are very bad at creating secure IMPI ports, but some people don't last time I checked, which was a few years ago. And someone compromising your system for that (e.g. the firewall in front of them, the VPN to it, or your systems you use to access it) could also get in.
If you use Debian, Ubuntu, Fedora and have encrypted the system partition you are potentially vunerable.
To exploit, reboot server, hold down enter key for 70+ seconds and you'll be dropped into a root shell.