A2: Analog Malicious Hardware
Kaiyuan Yang et al. (PDF) (via Brendan O’Connor):
While the move to smaller transistors has been a boon for performance it has dramatically increased the cost to fabricate chips using those smaller transistors. This forces the vast majority of chip design companies to trust a third party— often overseas—to fabricate their design. To guard against shipping chips with errors (intentional or otherwise) chip design companies rely on post-fabrication testing. Unfortunately, this type of testing leaves the door open to malicious modifications since attackers can craft attack triggers requiring a sequence of unlikely events, which will never be encountered by even the most diligent tester.
In this paper, we show how a fabrication-time attacker can leverage analog circuits to create a hardware attack that is small (i.e., requires as little as one gate) and stealthy (i.e., requires an unlikely trigger sequence before effecting a chip’s functionality).
In fact, researchers at the University of Michigan haven’t just imagined that computer security nightmare; they’ve built and proved it works. In a study that won the “best paper” award at last week’s IEEE Symposium on Privacy and Security, they detailed the creation of an insidious, microscopic hardware backdoor proof-of-concept. And they showed that by running a series of seemingly innocuous commands on their minutely sabotaged processor, a hacker could reliably trigger a feature of the chip that gives them full access to the operating system. Most disturbingly, they write, that microscopic hardware backdoor wouldn’t be caught by practically any modern method of hardware security analysis, and could be planted by a single employee of a chip factory.
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The “demonically clever” feature of the Michigan researchers’ backdoor isn’t just its size, or that it’s hidden in hardware rather than software. It’s that it violates the security industry’s most basic assumptions about a chip’s digital functions and how they might be sabotaged. Instead of a mere change to the “digital” properties of a chip—a tweak to the chip’s logical computing functions—the researchers describe their backdoor as an “analog” one: a physical hack that takes advantage of how the actual electricity flowing through the chip’s transistors can be hijacked to trigger an unexpected outcome.
