NSA's BIOS Backdoor a.k.a. God Mode Malware
Here's another reason why using a free BIOS like Coreboot is a great idea - if malware attempts to reflash it with an infected copy of presumably the original proprietary BIOS, it will be very, very obvious.Another "tamper detection" idea, if you're OK with using proprietary BIOS and/or Coreboot isn't supported, is to add a small piece of code to the existing BIOS that simply hashes the ROM contents and prints out a human-readable string to the boot screen based on that. If it goes missing or changes, then you can suspect something happened. Of course, the trick is to not have everyone doing this with the same code, or backdoors will just detect and spoof it. I modded the BIOS of an old machine I have to do this, it prints out "Customised by {my handle}".