Spectre (and Meltdown)
Beginner
A kind of attack, found in 2018, that tricks a processor’s guessing into leaving clues about secrets in its fast memory.
Novice
A family of attacks published in 2018. They make a processor speculatively run instructions that touch secret data; the wrong-path work is undone, but it leaves traces in the cache that can be measured. Meltdown is a related attack on out-of-order execution past a permission check.
Expert
Transient-execution attacks: mistrained predictors (Spectre) or delayed fault handling (Meltdown) let transient instructions encode secrets in microarchitectural state, read back through a cache timing side channel. Mitigations span hardware, OS (KPTI) and software (fences, process isolation).
Explained in Branch prediction and out-of-order execution (Architectures).
See also: Speculative execution, side-channel attack.