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Microprocessors from Intel, AMD, and other companies contain a newly discovered weakness that remote attackers can exploit to obtain cryptographic keys and other secret data traveling through the hardware, researchers said on Tuesday.
Hardware manufacturers have long known that hackers can extract secret cryptographic data from a chip by measuring the power it consumes while processing those values. Fortunately, the means for exploiting power-analysis attacks against microprocessors is limited because the threat actor has few viable ways to remotely measure power consumption while processing the secret material. Now, a team of researchers has figured out how to turn power-analysis attacks into a different class of side-channel exploit that's considerably less demanding.
Targeting DVFS
The team discovered that dynamic voltage and frequency scaling (DVFS)—a power and thermal management feature added to every modern CPU—allows attackers to deduce the changes in power consumption by monitoring the time it takes for a server to respond to specific carefully made queries. The discovery greatly reduces what's required. With an understanding of how the DVFS feature works, power side-channel attacks become much simpler timing attacks that can be done remotely.
The researchers have dubbed their attack Hertzbleed because it uses the insights into DVFS to expose—or bleed out—data that's expected to remain private. The vulnerability is tracked as CVE-2022-24436 for Intel chips and CVE-2022-23823 for AMD CPUs. The researchers have already shown how the exploit technique they developed can be used to extract an encryption key from a server running SIKE, a cryptographic algorithm used to establish a secret key between two parties over an otherwise insecure communications channel.
The researchers said they successfully reproduced their attack on Intel CPUs from the 8th to the 11th generation of the Core microarchitecture. They also claimed that the technique would work on Intel Xeon CPUs and verified that AMD Ryzen processors are vulnerable and enabled the same SIKE attack used against Intel chips. The researchers believe chips from other manufacturers may also be affected.
In a blog post explaining the finding, research team members wrote:
Hertzbleed is a new family of side-channel attacks: frequency side channels. In the worst case, these attacks can allow an attacker to extract cryptographic keys from remote servers that were previously believed to be secure.
Hertzbleed takes advantage of our experiments showing that, under certain circumstances, the dynamic frequency scaling of modern x86 processors depends on the data being processed. This means that, on modern processors, the same program can run at a different CPU frequency (and therefore take a different wall time) when computing, for example, 2022 + 23823 compared to 2022 + 24436.
Hertzbleed is a real, and practical, threat to the security of cryptographic software.
We have demonstrated how a clever attacker can use a novel chosen-ciphertext attack against SIKE to perform full key extraction via remote timing, despite SIKE being implemented as “constant time”.
Intel Senior Director of Security Communications and Incident Response Jerry Bryant, meanwhile, challenged the practicality of the technique. In a post, he wrote: "While this issue is interesting from a research perspective, we do not believe this attack to be practical outside of a lab environment. Also note that cryptographic implementations that are hardened against power side-channel attacks are not vulnerable to this issue." Intel has also released guidance here for hardware and software makers.
Neither Intel nor AMD are issuing microcode updates to change the behavior of the chips. Instead, they're endorsing changes Microsoft and Cloudflare made respectively to their PQCrypto-SIDH and CIRCL cryptographic code libraries. The researchers estimated that the mitigation adds a decapsulation performance overhead of 5 percent for CIRCL and 11 percent for PQCrypto-SIDH. The mitigations were proposed by a different team of researchers who independently discovered the same weakness.
AMD declined to comment ahead of the lifting of a coordinated disclosure embargo.
At the granularity of milliseconds
In explaining the Hertzbleed attack, the researchers wrote:
In this paper, we show that, on modern Intel (and AMD) x86 CPUs, power-analysis attacks can be turned into timing attacks—effectively lifting the need for any power measurement interface. Our discovery is enabled by the aggressive dynamic voltage and frequency scaling (DVFS) of these CPUs. DVFS is a commonly-used technique that consists of dynamically adjusting CPU frequency to reduce power consumption (during low CPU loads) and to ensure that the system stays below power and thermal limits (during high CPU loads). We find that, under certain circumstances, DVFS-induced CPU frequency adjustments depend on the current power consumption at the granularity of milliseconds. Therefore, since the power consumption is data dependent, it follows transitively that CPU frequency adjustments are data dependent too.
Making matters worse, we show that data-dependent frequency adjustments can be observed without the need for any special privileges and even by a remote attacker. The reason is that CPU frequency differences directly translate to execution time differences (as 1 hertz = 1 cycle per second). The security implications of this finding are significant. For example, they fundamentally undermine constant-time programming, which has been the bedrock defense against timing attacks since their discovery in 1996 [58]. The premise behind constant-time programming is that by writing a program to only use “safe” instructions, whose latency is invariant to the data values, the program’s execution time will be data-independent. With the frequency channel, however, timing becomes a function of data—even when only safe instructions are used.
Despite its theoretical power, it is not obvious how to construct practical exploits through the frequency side channel. This is because DVFS updates depend on the aggregate power consumption over millions of CPU cycles and only reflect coarse-grained program behavior. Yet, we show that the frequency side channel is a real threat to the security of cryptographic software, by (i) reverse engineering a precise leakage model for this channel on modern x86 CPUs, and (ii) showing that some cryptographic primitives admit amplification of single key bit guesses into thousands of high- or low-power operations, enough to induce a measurable timing difference.
Riccardo Paccagnella, a University of Illinois Urbana-Champaign researcher and a co-author of the paper, said that Hertzbleed demonstrates the obsolescence of guidance jointly hammered out by hardware and software engineers for writing software that isn't susceptible to timing attacks. "The result is that current industry guidelines for how to write constant-time code (such as Intel's one) are insufficient to guarantee constant-time execution on modern processors," he wrote in an online message.
For now, there's nothing end-users can do, and even if there was, it's not clear at this point that Hertzbleed represents a clear and present threat. Instead, developers should carefully consider how the findings affect the security of the cryptographic software they design. The researchers propose other methods for hardening apps against Hertzbleed-like attacks.
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