Dozens of popular websites, including Twitter and Netflix, unreachable in the US for parts of the day on Friday, as attackers controlling a vast collection of internet devices unleashed several massive attacks, a report said.

Web-technology provider Dynamic Network Services (Dyn) said its domain-name-system services were subject to a massive denial-of-service attack starting at 7:10 a.m. EDT on Friday, reported the Wall Street Journal.

After the first onslaught was resolved, Dyn reported another wave of attacks that caused disruptions through the day. Distributed denial-of-service attacks can knock websites offline by flooding them with junk data, blocking the way for legitimate users, according to the report.

Dyn chief strategy officer Kyle York said the attack came from “tens of millions” of addresses on machines that had been infected with malicious software code. The code—known as Mirai—takes advantage of a weakness in internet-connected devices and forms them into a collection of attacking machines, called a “botnet.”

Other sites temporarily disrupted by the attacks included PayPal Holdings, Shopify, Airbnb, Kayak and GitHub, a service used by programmers and major technology firms to create software, the report said.