https://techcrunch.com/2018/05/21/intel-discloses-a-new-spectre-exploit-variant-but-leaves-mitigation-off-by-default/

The specter of Spectre still looms above chipmakers; a new variant of that most dire of chip flaws was disclosed today, and Intel has a patch ready to go. It’s issuing the mitigation in tandem with the announcement that may come with a serious performance hit — which is why it will be off by default.

Like the other Spectre variants, this one has to do with “speculative execution,” a core component of modern computing architecture that predicts what might be required of it in the immediate future and executes on it, either keeping the results if the prediction is right or discarding them if not. Spectre variants basically trick the processor into revealing the data it uses for speculative execution, potentially allowing an attacker to get at even highly protected bits. Unlike Meltdown, which affected Intel primarily, Spectre affects other chip manufacturers as well.