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Intel, Google, and Microsoft today disclosed a new variant of the Spectre design flaw and security vulnerability that impacts millions of computers and mobile devices from a range of manufacturers.

Called Variant 4, or the Speculative Store Bypass, the vulnerability is similar to Spectre, taking advantage of the speculative execution mechanism of a CPU to allow hackers to gain access to sensitive information. Variant 4 was demonstrated by researchers in a language-based runtime environment.

CVE-2018-3639 – Speculative Store Bypass (SSB) – also known as Variant 4

Systems with microprocessors utilizing speculative execution and speculative execution of memory reads before the addresses of all prior memory writes are known may allow unauthorized disclosure of information to an attacker with local user access via a side-channel analysis.

According to Intel, the new vulnerability has a “moderate” severity rating because many of the exploits that it uses have already been addressed through mitigations that were first introduced by software makers and OEMs in January for Meltdown and Spectre. Intel is, however, releasing a full mitigation option that will “prevent this method from being used in other ways.”

This additional mitigation for Variant 4 has been delivered in beta form to OEM system manufacturers and system software vendors, and Intel is leaving it up to its partners to decide whether or not to implement the extra measures. Intel plans to leave the mitigation set to off by default because of the potential for performance issues.

This mitigation will be set to off-by-default, providing customers the choice of whether to enable it. We expect most industry software partners will likewise use the default-off option. In this configuration, we have observed no performance impact. If enabled, we’ve observed a performance impact of approximately 2 to 8 percent based on overall scores for benchmarks like SYSmark(R) 2014 SE and SPEC integer rate on client1 and server2 test systems.

The Spectre and Meltdown family of vulnerabilities affect all modern processors from Intel, ARM, and AMD, but Intel has faced more scrutiny over the design flaw due to its high-profile position in the processor market. Apple’s iOS and Mac devices are affected by these vulnerabilities, but Apple has historically been quick to patch them.

Prior to when Spectre and Meltdown were initially discovered, for example, Apple had already implemented some patches and has since addressed known Meltdown and Spectre vulnerabilities with little impact to performance on Macs or iOS devices. As mentioned above, many of the exploits in Variant 4 have been previously addressed by Apple and other manufacturers in already-existing software patches.

Spectre and Meltdown-related vulnerabilities are hardware-based and therefore must be mitigated rather than outright fixed, but future Intel chips will not be as vulnerable. Intel has said that its next-generation Xeon Scalable processors (Cascade Lake) and its 8th-generation Intel Core processors will feature redesigned components to protect against some Spectre and Meltdown flaws.

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