A number of issues relating to speculative execution were found last year and publicly announced January 3rd. Two of these, known as Meltdown and Spectre V2, are addressed here.
CVE-2017-5754 (Meltdown)
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This issue relies on an affected CPU speculatively executing instructions beyond a faulting instruction. When this happens, changes to architectural state are not committed, but observable changes may be left in micro-architectural state (for example, cache). This may be used to infer privileged data.
CVE-2017-5715 (Spectre V2)
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Spectre V2 uses branch target injection to speculatively execute kernel code at an address under the control of an attacker.