New Variant of Spectre Attack Bypasses Intel and Arm Hardware Mitigations

A team of researchers from the Vrije Universiteit Amsterdam in the Netherlands has demonstrated a new Spectre attack variant that can bypass hardware mitigations implemented in recent years by Intel and Arm.

Since the disclosure of the Spectre and Meltdown vulnerabilities back in January 2018, researchers have continued looking into the security of processors and they have found several other side-channel attack methods. These methods can typically allow an attacker with local access to the targeted machine to obtain small bits of potentially sensitive data from memory.

When the Spectre vulnerability was found, the most dangerous variant was called Spectre v2 or Spectre BTI (Branch Target Injection). Affected CPU makers, such as Intel and Arm, have been developing hardware mitigations to prevent these types of exploits.

However, VU Amsterdam researchers this week disclosed the details of what they have described as an “extension of Spectre v2.” The new variant, dubbed Branch History Injection (BHI) and Spectre-BHB, bypasses those hardware mitigations. Another slightly different variant uncovered by the researchers is called Intra-mode BTI (IMBTI).

“The mitigations [implemented by Intel and Arm] work as intended, but the residual attack surface is much more significant than vendors originally assumed,” the researchers explained.

They demonstrated their findings by creating what they described as a “neat end-to-end exploit leaking arbitrary kernel memory on modern Intel CPUs.” They have also released a video showing the exploit in action.

Intel has published an advisory and a technical document describing the new vulnerabilities, which the chipmaker tracks as CVE-2022-0001 and CVE-2022-0002. The flaws have been assigned a severity rating of “medium.”

Intel says future processors are expected to include hardware mitigations for BHI attacks, and in the meantime it has described several software mitigation options.

Arm has published an advisory, as well as an FAQ, a knowledge base article, and a paper describing the vulnerability and mitigations. Arm tracks the Spectre-BHB vulnerability as CVE-2022-23960.

AMD processors do not appear to be affected by Spectre-BHB. However, researchers at grsecurity this week disclosed the details of a vulnerability affecting AMD CPUs. The issue, tracked as CVE-2021-26341, is related to speculative behavior of branch instructions, and it can result in data leakage. AMD has published an advisory for CVE-2021-26341, as well as a white paper detailing software techniques for managing speculation on its processors.

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