Meltdown
CPU vulnerability (CVE-2017-5754, Jan 2018) that let a userspace process read kernel memory via speculative execution + cache-timing leakage.
Specific to Intel: the pipeline let privileged reads bypass permission checks until the result would eventually commit — but by then the address contents had already modified the cache, measurable from the outside. KPTI (Kernel Page Table Isolation) in Linux became the fix, with a 5–30 % performance hit.
Together with Spectre it opened the category "microarchitectural side channels" — since then we've seen MDS, L1TF, Retbleed, Inception, Downfall, ZenBleed. CPU security suddenly became a mainstream concern.