On Wednesday, at about 12:15 pm EST, 1.35 terabits per second of traffic hit the developer platform GitHub all at once. It was the most powerful distributed denial of service attack recorded to date—and it used an increasingly popular DDoS method, no botnet required.

GitHub briefly struggled with intermittent outages as a digital system assessed the situation. Within 10 minutes it had automatically called for help from its DDoS mitigation service, Akamai Prolexic. Prolexic took over as an intermediary, routing all the traffic coming into and out of GitHub, and sent the data through its scrubbing centers to weed out and block malicious packets. After eight minutes, attackers relented and the assault dropped off.

The scale of the attack has few parallels, but a massive DDoS that struck the internet infrastructure company Dyn in late 2016 comes close. That barrage peaked at 1.2 terabits per second and caused connectivity issues across the US as Dyn fought to get the situation under control.

“We modeled our capacity based on fives times the biggest attack that the internet has ever seen,” Josh Shaul, vice president of web security at Akamai told WIRED hours after the GitHub attack ended. “So I would have been certain that we could handle 1.3 Tbps, but at the same time we never had a terabit and a half come in all at once. It’s one thing to have the confidence. It’s another thing to see it actually play out how you’d hope."

Real-time traffic from the DDoS attack. Akamai

Akamai defended against the attack in a number of ways. In addition to Prolexic's general DDoS defense infrastructure, the firm had also recently implemented specific mitigations for a type of DDoS attack stemming from so-called memcached servers. These database caching systems work to speed networks and websites, but they aren't meant to be exposed on the public internet; anyone can query them, and they'll likewise respond to anyone. About 100,000 memcached servers, mostly owned by businesses and other institutions, currently sit exposed online with no authentication protection, meaning an attacker can access them and send them a special command packet that the server will respond to with a much larger reply.

Unlike the formal botnet attacks used in large DDoS efforts, like against Dyn and the French telecom OVH, memcached DDoS attacks don't require a malware-driven botnet. Attackers simply spoof the IP address of their victim and send small queries to multiple memcached servers—about 10 per second per server—that are designed to elicit a much larger response. The memcached systems then return 50 times the data of the requests back to the victim.