Researchers have discovered a new type of side-channel attack called Hertzbleed, which allows remote attackers to extract cryptographic keys by analyzing changes in processor frequency. The attack is made possible by the dynamic voltage and frequency scaling (DVFS) mechanism used in modern Intel (CVE-2022-24436) and AMD (CVE-2022-23823) CPUs.
DVFS is used to save energy and prevent overheating: the processor reduces or increases the frequency depending on the load. However, it turned out that frequency fluctuations depend not only on power, but also on the data being processed. Scientists from the University of Texas at Austin, the University of Illinois at Urbana-Champaign, and the University of Washington were able to exploit this feature.
According to the researchers, “in the worst case, the attack allows cryptographic keys to be extracted from remote servers that were considered secure. Hertzbleed is a real and practical threat to cryptographic software.”
The main danger is that the attack does not require direct access to the power consumption interface: frequency analysis can be converted into remote timing analysis. Moreover, even correctly implemented “time-constant” cryptographic code can leak through such measurements.
Intel has confirmed that the vulnerability affects all of its processors and can be exploited remotely with fairly sophisticated attacks that do not require user interaction. AMD has also acknowledged the problem — it affects devices based on Zen 2 and Zen 3 microarchitectures (desktop, mobile, server CPUs, and Chromebooks).
Researchers do not rule out that processors from other manufacturers, including ARM, may also be at risk if they use similar dynamic frequency change mechanisms. However, there is no evidence of PoC being applied to these devices yet.
Intel and AMD do not plan to release microcode updates to eliminate frequency side channel attacks. Instead, manufacturers recommend that developers use masking, hiding, or key rotation techniques to reduce the risk of data leakage.
One way to protect against this is to disable the frequency overclocking feature: Turbo Boost for Intel and Turbo Core or Precision Boost for AMD. However, experts note that this significantly reduces performance and does not guarantee complete protection.
Detailed recommendations for developers are published by Intel in its guide to secure cryptography implementation. A study titled “Hertzbleed: Turning Power Side-Channel Attacks into Remote Timing Attacks on x86” will be presented at the USENIX Security Symposium in Boston (August 10-12, 2022).