Securing Macs against stealthy malware infections could get more complicated thanks to a new proof-of-concept exploit that allows attackers with brief physical access to covertly replace the firmware of most machines built since 2011.

Once installed, the bootkit—that is, malware that replaces the firmware that is normally used to boot Macs—can control the system from the very first instruction. That allows the malware to bypass firmware passwords, passwords users enter to decrypt hard drives and to preinstall backdoors in the operating system before it starts running. Because it’s independent of the operating system and hard drive, it will survive both reformatting and OS reinstallation. And since it replaces the digital signature Apple uses to ensure only authorized firmware runs on Macs, there are few viable ways to disinfect infected boot systems. The proof-of-concept is the first of its kind on the OS X platform. While there are no known instances of bootkits for OS X in the wild, there is currently no way to detect them, either.

The malware has been dubbed Thunderstrike, because it spreads through maliciously modified peripheral devices that connect to a Mac’s Thunderbolt interface. When plugged into a Mac that’s in the process of booting up, the device injects what’s known as an Option ROM into the extensible firmware interface (EFI), the firmware responsible for starting a Mac’s system management mode and enabling other low-level functions before loading the OS. The Option ROM replaces the RSA encryption key Macs use to ensure only authorized firmware is installed. From there, the Thunderbolt device can install malicious firmware that can’t easily be removed by anyone who doesn’t have the new key.