Hackers Can Mess With Voltages to Steal Intel Chips' Secrets

When thieves want to steal treasures surrounded by sensors and alarms, they sometimes resort to cutting the power, disrupting the flow of electricity to those expensive security systems. It turns out that hackers can pull off a similar trick: breaking the security mechanisms of Intel chips by messing with their power supply, and exposing their most sensitive secrets.Two teams of researchers—one at the University of Birmingham in the UK, TU Graz in Vienna, KU Leuven in Belgium and another at the Technische Universität Darmstadt in Germany and the University of California—have found a new technique that can allow hackers to fiddle with the voltage of Intel chips to cause them to leak information stored using Intel's Secure Guard Extensions feature. Those "secure enclaves" in a device's memory are designed to be impregnable. Intel, which asked the teams to keep their findings under wraps for the last six months, confirmed the findings and pushed out an update to its chip firmware to prevent the attack today.
The technique, which one of the two teams calls Plundervolt, involves planting malicious software on a target computer that temporarily reduces the voltage of the electricity flowing to an Intel chip. That drop in voltage, known as "undervolting," typically allows legitimate users to save power when they don't need maximum performance. (By that same token, you can use the voltage-variance feature to "overclock" a processor for more intensive tasks.) But by momentarily undervolting a processor by 25 or 30 percent, and precisely timing that voltage change, an attacker can cause the chip to make errors in the midst of computations that use secret data. And those errors can reveal information as sensitive as a cryptographic key or biometric data stored in the SGX enclave.

"Writing to memory takes power," says Flavio Garcia, a computer scientist at the University of Birmingham who, along with his colleagues, will present the Plundervolt research at IEEE Security and Privacy next year. "So for an instant, you reduce the CPU voltage to induce a computation fault."Once the researchers found that they could use voltage changes to induce those faults—a so-called fault injection or "bit flip" that turns a one to a zero in the SGX enclave or vice versa—they showed that they could also exploit them. "If you can flip bits when, for instance, you're doing cryptographic computations—and that's where this gets interesting—you can recover the secret key," Garcia says. In many cases, the researchers explain, changing a single bit of a cryptographic key can make it vastly weaker, so that an attacker can both decipher the data it encrypts and derive the key itself. You can see the impact on an AES encryption key here:

The researchers also showed that they could use those bit flips to make the processor write to an unprotected portion of memory rather than to the secure SGX enclave:

The researchers acknowledge that their attack isn't exactly easy to pull off. For it to work, the attacker has to have already somehow installed their malware with high-level, or "root," privileges on the target computer. But Intel has advertised its SGX feature as preventing corruption or theft of sensitive data even in the face of this sort of highly privileged malware. The researchers say they have demonstrated a serious exception to that guarantee.