The botnets are made up of tens of thousands of Internet of Things (IoT) devices, including unsecure routers, digital video recorders (DVRs) and connected IP cameras. Such IoT machines have been shown widely vulnerable to simple hacks, meaning the bot masters are easily able to build up vast networks of compromised systems to send extraordinary volumes of traffic to a chosen target.
Source : How Hacked Cameras Are Helping Launch The Biggest Attacks The Internet Has Ever Seen