Attackers can infect MacBook computers with highly persistent boot rootkits by connecting malicious devices to them over the Thunderbolt interface.

The attack, dubbed Thunderstrike, installs malicious code in a MacBook’s boot ROM (read-only memory), which is stored in a chip on the motherboard. It was devised by a security researcher named Trammell Hudson based on a two-year old vulnerability and will be demonstrated next week at the 31st Chaos Communication Congress in Hamburg.

“It is possible to use a Thunderbolt Option ROM to circumvent the cryptographic signature checks in Apple’s EFI firmware update routines,” Hudson said in the description of his upcoming 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.”

Malicious code installed in the MacBook boot ROM will be executed before the OS is loaded, meaning it can patch the OS kernel and have complete control over the system. It also means that reinstalling Mac OS X will not remove the bootkit and neither will replacing the hard disk drive, because the malicious code is not stored on it.

The bootkit can even replace Apple’s cryptographic key stored in the ROM with one generated by the attacker, preventing any future legitimate firmware updates from Apple, the researcher said in a blog post.

Firmware updates are supposed to be signed, but the vulnerability exploited by this attack allows that mechanism to be bypassed.

“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,” the researcher said. “The devices remain functional, which would allow a stealthy bootkit to spread across air-gap security perimeters through shared Thunderbolt devices.”

This worm-like spreading capability is similar to that of BadUSB, a stealthy malware attack demonstrated earlier this year at the Black Hat security conference that can infect the firmware of USB devices and then use them to compromise other computers.

Security researchers have also previously demonstrated methods to bypass Secure Boot, a security mechanism of the Unified Extensible Firmware Interface (UEFI)—the BIOS replacement in modern computers—in order to install bootkits.