An attack on MacBooks, called Thunderstrike, has been uncovered that makes use of the laptops’ physical Thunderbolt interface to achieve persistent boot rootkits.
Security researcher Trammell Hudson is prepping a demonstration of the attack for the 31st Chaos Communication Congress in Hamburg. It’s based on a two-year old vulnerability, and will show how hackers can use malicious code to infect a MacBook's boot read-only memory (ROM), which is stored in a chip on the motherboard.
This results in the installation of persistent firmware modifications into the EFI boot ROM of Apple's popular MacBooks. The ROM is executed before the OS is loaded, so that it can simply hijack the OS kernel and take over the system. That also means that updating Mac OS X and/or replacing the hard disk drive will have no effect on the malware.
The bootkit can be easily installed by an evil-maid via the externally accessible Thunderbolt ports. Once installed, it can prevent software attempts to remove it and could spread virally across air-gaps by infecting additional Thunderbolt devices.
"It is possible to use a Thunderbolt Option ROM to circumvent the cryptographic signature checks in Apple's EFI firmware update routines," Hudson said, describing his presentation. "This allows an attacker with physical access to the machine to write untrusted code to the SPI flash ROM on the motherboard and creates a new class of firmware bootkits for the MacBook systems."
Thunderstrike also has a clever replication technique. "Additionally, other Thunderbolt devices' Option ROMs are writable from code that runs during the early boot and the bootkit could write copies of itself to new Thunderbolt devices," he said. "The devices remain functional, which would allow a stealthy bootkit to spread across air-gap security perimeters through shared Thunderbolt devices."