Microsoft and Google are jointly disclosing a new CPU security vulnerability that’s similar to the Meltdown and Spectre flaws that were revealed earlier this year. Labelled Speculative Store Bypass (variant 4), the latest vulnerability is a similar exploit to Spectre and exploits speculative execution that modern CPUs use. Browsers like Safari, Edge, and Chrome were all patched for Meltdown earlier this year, and Intel says “these mitigations are also applicable to variant 4 and available for consumers to use today.”

However, unlike Meltdown (and more similar to Spectre) this new vulnerability will also include firmware updates for CPUs that could affect performance. Intel has already delivered microcode updates for Speculative Store Bypass in beta form to OEMs, and the company expects them to be more broadly available in the coming weeks. The firmware updates will set the Speculative Store Bypass protection to off-by-default, ensuring that most people won’t see negative performance impacts.

“If enabled, we’ve observed a performance impact of approximately 2-8 percent based on overall scores for benchmarks like SYSmark 2014 SE and SPEC integer rate on client 1 and server 2 test systems,” explains Leslie Culbertson, Intel’s security chief.

As a result, end users (and particularly system administrators) will have to pick between security or optimal performance. The choice, like previous variants of Spectre, will come down to individual systems and servers, and the fact that this new variant appears to be less of a risk than the CPU flaws that were discovered earlier this year.

Microsoft started offering up to $250,000 for bugs that are similar to the Meltdown and Spectre CPU flaws in March, and the company says it discovered this new bug back in November. “Microsoft previously discovered this variant and disclosed it to industry partners in November of 2017 as part of Coordinated Vulnerability Disclosure (CVD),” says a Microsoft spokesperson. Microsoft is now working with Intel and AMD to determine performance impacts on systems.

“We are continuing to work with affected chip manufacturers and have already released defense-in-depth mitigations to address speculative execution vulnerabilities across our products and services,” says a Microsoft spokesperson. “We’re not aware of any instance of this vulnerability class affecting Windows or our cloud service infrastructure. We are committed to providing further mitigations to our customers as soon as they are available, and our standard policy for issues of low risk is to provide remediation via our Update Tuesday schedule.”

Intel is already preparing its own CPU changes for the future. Intel is redesigning its processors to protect against attacks like Spectre or this new variant 4, and the company’s next-generation Xeon processors (Cascade Lake) will include new built-in hardware protections, alongside 8th generation Intel Core processors that ship in the second half of 2018.