In August and September, threat actors unleashed the biggest distributed denial-of-service attacks in Internet history by exploiting a previously unknown vulnerability in a key technical protocol. Unlike other high-severity zerodays in recent years—Heartbleed or log4j, for example—which caused chaos from a torrent of indiscriminate exploits, the more recent attacks, dubbed HTTP/2 Rapid Reset, were barely noticeable to all but a select few engineers.

Source: Technology Lab – Ars Technica