It is definitely not something to boast about, but it is certainly security information that we Mac users need to know.
According Hudson Trammel, a security researcher who attended the Chaos Computer Congress in Hamburg (Germany), the firmware on some Ma computers can be rewritten using a ROM-loaded Thunderbolt device (read only memory, or read-only memory) special replacement.
Lecture by Trammel Hudson at Chaos Computer Congress.
Nicknamed "Thunderstrike", O hack cannot be detected once installed. More than that, it cannot be removed neither in a possible reinstallation of OS X nor replacing the hard disk, since it affects the boot ROM (which is something totally independent) and replaces the public RSA key (data encryption algorithm) of Apple. In addition, when making this replacement, it blocks future firmware updates, unless the update has the signature of the key used by the hacker.
Do you think it's over? No. O hack you can even copy to ROMs (during a reboot) from other Thunderbolt devices connected to a compromised Mac. Apparently the new Mac mini and iMacs with Retina 5K screen have partial protection for this type of attack, it remains to be seen whether Apple will take this to other Mac models.
The good news is that the hacker needs to have physical access to the machine to be able to do this. In addition, Hudson is in contact with Apple.
Anyone who likes the subject and wants to know more information about the procedure, is worth reading Hudson's article or watching the video of the lecture.