Cloudflare, Google, Microsoft, and Amazon all say they successfully mitigated what two of the companies called the biggest DDoS layer 7 attacks they’ve recorded in August and September, though none said who the attacks were directed against. The companies say the attacks were possible because of a zero-day vulnerability in the HTTP/2 protocol they’ve named “HTTP/2 Rapid Reset.”

HTTP/2 speeds up page loading by allowing for multiple simultaneous requests to a website over a single connection. Cloudflare writes that these attacks apparently involved an automated cycle of sending and immediately canceling “hundreds of thousands” of requests to websites that use HTTP/2, overwhelming servers and taking them offline.

Google recorded the heaviest assault at over 398 million requests per second, which it says is more than seven times larger than any such attack it has recorded before. (The record was last held by a 2022 attack that “peaked at 46 million requests per second.”) Cloudflare saw 201 million requests per second at the peak, which it also called record-breaking, while Amazon recorded the fewest requests, maxing out at 155 million per second. Microsoft did not disclose its own figures.

DDoS attacks are common — in June, Microsoft reported a large-scale layer 7 attack that downed Outlook for thousands of its users. The same month, fan-fiction website AO3 was also affected by DDoS attacks. A group called Anonymous Sudan claimed credit for both attacks.

Google goes into detail in a blog post about how the attacks worked, so do head over there if you want to roll your sleeves up and read about it.

Update October 10th, 2023, 1:20PM ET: Added that Microsoft has disclosed that its cloud infrastructure was affected as well.