http://www.techradar.com/news/heres-the-latest-security-bug-your-computer-could-be-exposed-by

A newly discovered firmware vulnerability could leave countless Windows and Mac computers at risk from a hack, according to security researchers from Duo Labs. The vulnerability could be used by malware to gain deep access to systems.

The bug involves the extensible firmware interface, or EFI, which is the first bit of code that runs when you hit the power button - part of its responsibilities include validating the software that's running on the machine.