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.

Enter evil maid

While the hack requires an attacker to have brief physical access to a targeted machine, that prerequisite isn't prohibitively steep in many situations. For example, so-called "evil maid" scenarios—in which a rogue hotel housekeeper tampers with a computer—or an agent at an international border crossing both routinely have access to computers, often while unsupervised. Documents leaked by former National Security Agency subcontractor Edward Snowden also exposed how agents intercept hardware being shipped to organizations targeted for surveillance and covertly install modified firmware onto them before they’re delivered.

All any of these attackers would need to do to carry out a Thunderstrike-style attack is to reboot a Mac with a previously weaponized Thunderbolt device attached. If the machine is turned on but locked, the attacker need only press the power button for a few seconds to hard-reboot the machine. Firmware passwords, disk encryption passwords, and user passwords won't thwart the attack since the Option ROMs are loaded before any of those protections are checked.

Thunderstrike made its debut in late December, at the Chaos Communication Congress. The vulnerability was discovered by Trammell Hudson, an employee of a high-tech hedge fund in New York City called Two Sigma Investments, while trying to secure the firm's MacBooks. A self-described reverse engineering hobbyist, Hudson was previously known for creating Magic Lantern, an open source programming environment for Canon digital SLR cameras.

Thunderstrike builds on a similar attack as demonstrated at the 2012 Blackhat conference that bypasses OS X FileVault protections to install a rootkit. Like Thunderstrike, the 2012 exploit used Thunderbolt ports to inject the malicious payload into the boot process, but the earlier attack wasn't able to modify the boot ROM itself. To work around that limitation, the researcher—who works under the hacking moniker snare—wrote the bootkit to the EFI system partition.

Eureka

One of the breakthroughs of Thunderstrike is its ability to get the boot ROM firmware volumes validated. Hudson figured out how to do this after discovering an undocumented CRC32 cyclic redundancy check routine carried out during the normal validation process. A second breakthrough involved the discovery that Option ROMs are loaded during a recovery mode boot. That allowed Hudson to figure out how to replace Apple's existing EFI code.

Thunderstrike was just one of at least two EFI-based attacks that were demonstrated at December's Chaos Communication Congress. A separate talk delved into the Unified Extensible Firmware Interface, a similar mechanism that's used to boot some Windows and Linux machines. Hudson said an attack technique known as Dark Jedi that was outlined during the talk could possibly be adapted to make his exploit work remotely, so the attacker wouldn't require physical access. Earlier this week, the US CERT issued three advisories warning of vulnerabilities in widely used UEFI chips. A researcher from security firm Bromium also has this brief writeup on the UEFI talk.

Hudson said Apple is in the process of partially patching the vulnerabilities that make Thunderstrike possible. The remedy involves not allowing Option ROMs to load during firmware updates, a measure that Hudson said is effective against his current proof of concept. Apple already has begun rolling out the upgrade to Mac Mini's and iMac Retina 5ks and plans to make it more widely available soon.

"However... it is not a complete fix," he warned in a blog post detailing Thunderstrike. "Option ROMs are still loaded on normal boots, allowing snare's 2012 attack to continue working. Older Macs are subject to downgrade attacks by 'updating' to a vulnerable firmware version."

Until there's a complete fix from Apple, there aren't a lot of viable options for preventing Thunderstrike-type attacks. Pouring a liberal amount of epoxy glue in a Thunderbolt port will certainly make the exploit harder, since it would force an attacker to take apart the casing to access the underlying flash ROM chip, but it would come at the cost of disabling key functionality. The other obvious solution is for people to keep their machines on their person at all times, but that isn't always practical, either. Hotel safes and locked and sealed storage boxes are also only partially effective, since both measures are vulnerable to cracking and picking.