Thunderbolt Vulnerabilities Could Threaten Millions of PCs

Security researchers have discovered Thunderbolt vulnerabilities that could leave millions of computers exposed to attackers who have the right hardware tools and a few minutes with the machine. The "Thunderspy" attack affects Windows and Linux devices manufactured pre-2019.

Björn Ruytenberg, researcher with Eindhoven University of Technology, says the team found seven vulnerabilities in Intel's Thunderbolt port design and created nine attack scenarios in which someone could bypass defenses and read and copy data on a target machine. This tactic is effective against a computer that has an encrypted hard drive and is locked or set to sleep.

An attacker would need physical access to a Thunderbolt-enabled Windows or Linux system, a screwdriver, and a few off-the-shelf tools to pull this off. Even so, the attack is stealth, meaning a victim would not notice any traces of activity and it does not require any user interaction. 

"Thunderspy works even if you follow best security practices by locking or suspending your computer when leaving briefly, and if your system administrator has set up the device with Secure Boot, strong BIOS and operating system account passwords, and enabled full disk encryption," Ruytenberg explains in a blog post on the findings. "All the attacker needs is 5 minutes alone with the computer, a screwdriver, and some easily portable hardware."

This isn't the first security issue involving Thunderbolt technology, which has prompted a few concerns since it was introduced in 2011. Thunderbolt claimed to accelerate the speed of data transfer to external tools, which it did with Direct Memory Access (DMA)-enabled I/O system. In this kind of "evil maid" attack, Ruytenberg says, Thunderbolt has proven to be a "viable entry point in stealing data from encrypted drives and reading and writing all of system memory."

In early 2019, a group of vulnerabilities known as Thunderclap demonstrated how plugging a malicious peripheral device into a Thunderbolt port could compromise security of a target machine. An attacker could exploit the flaws to bypass security measures and run arbitrary code at the highest level of privilege. Following Thunderclap, Intel introduced Security Levels, an architecture designed to enable users to only authorize trusted Thunderbolt devices.

Unlike Thunderclap, the Thunderspy attacks break both Thunderbolt hardware and protocol security, Ruyterberg explains. Using the "evil maid" threat model, in which the attacker has physical access, as well as varying Security Levels, researchers created arbitrary Thunderbolt device identities, cloned user-authorized Thunderbolt devices, and obtained PCIe connectivity to conduct DMA attacks. They were able to disable Thunderbolt security entirely, he notes. The Thunderspy attacks reportedly break all primary security claims for Thunderbolt 1, 2, and 3.

All Thunderbolt-equipped systems shipped between 2011-2020 are exposed; however, some systems are only partially vulnerable. The flaws can't be fixed with software, he adds. They will affect future standards such as the Thunderbolt 4 and USB 4, and they require silicon redesign.

Devices that are at least partially protected include those manufactured with Kernel DMA protections, a security measure created by Intel after the Thunderclap flaws were disclosed. Ruytenberg notes Kernel DMA protection mitigates some, but not all, of the Thunderspy vulnerabilities. However, as Wired points out, Kernel DMA is not present in machines manufactured before 2019 and it is still not the standard now. Devices running macOS are partially affected by Thunderspy, Ruytenberg explains in a technical writeup of the vulnerabilities.

Ruyterberg shared his findings with Intel in February. Intel confirmed the vulnerabilities and today published a blog post with additional information. It advises following standard security practices, including use of only trusted peripherals and blocking unauthorized physical access.

Intel also notes that major operating systems implemented Kernel DMA protection in 2019. These include Windows (Windows 10 1803 RS4 and later), Linus (kernel 5.x and later), and macOS (macOS 10.12.4 and later). It says researchers did not demonstrate successful DMA attacks against systems with these protections enabled. Still, Microsoft notes that its own protections don't block DMA attacks via 1394/FireWire, PCMCIA, CardBus, and ExpressCard.

Should You Be Worried?

While it's true millions of devices could be affected by these vulnerabilities, industry watchers say chances of exploitation are slim. "On the scope of potential threats, this one is relatively minor," says Jack Gold, founder and principal analyst at J.Gold Associates. An attacker would need access to the target machine and to plug into the port, making this a very targeted effort.

"Most viruses are spread by emails, phishing attacks, things of that nature," he continues. "This is a hard thing to spread." It's unappealing to an attacker hoping for high value and little work. There are many other exploits and attack methods available to cybercriminals seeking large amounts of data and financial gain. For most, this level of planning wouldn't be worth it.

That said, Thunderspy could prove useful to an adversary who knows what they're after and who they need to target. "It's hard to see where there's real benefit to people doing this unless you know there's a machine that has information you need … someone who has specific valuable information [you] want to get to," Gold explains. For CEOs, CFOs, and other high-value targets, a solution could be an upgrade to a device with Kernel DMA protection in place, or they could simply keep a close eye on their machines.

Those whose devices are lost or stolen, however, may have greater cause for concern. Thunderspy can crack password-protected laptops, which could put a great deal of sensitive data at risk if they fall into the wrong hands.

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