The EFI component in Apple OS X before 10.11 allows physically proximate attackers to modify firmware during the EFI update process by inserting an Apple Ethernet Thunderbolt adapter with crafted code in an Option ROM, aka a "Thunderstrike" issue. NOTE: this issue exists because of an incomplete fix for CVE-2014-4498.
https://trmm.net/Thunderstrike_FAQ
https://support.apple.com/HT205267
http://www.securitytracker.com/id/1033703
http://lists.apple.com/archives/security-announce/2015/Sep/msg00008.html