Disclosed earlier this year, potentially dangerous Meltdown and Spectre vulnerabilities that affected a large family of modern processors proven that speculative execution attacks can be exploited in a trivial way to access highly sensitive information.
Since then, several more variants of speculative execution attacks have been discovered, including Spectre-NG, SpectreRSB, Spectre 1.1, Spectre1.2, TLBleed, Lazy FP, NetSpectre and Foreshadow, patches for which were released by affected vendors time-to-time.
Speculative execution is a core component of modern processors design that speculatively executes instructions based on assumptions that are considered likely to be true. If the assumptions come out to be valid, the execution continues, otherwise discarded.
Source: The Hacker News