Distributed denial-of-service (DDoS) attacks

A DDoS attack is a malicious attempt to disrupt the legitimate user traffic of a server by overwhelming it with a flood of random traffic. DDoS differs from DoS by its distributed nature, attacking a target from several independent networks of compromised systems. These compromised computer systems are called bots, and a botnet refers to a group of such bots under the control of the same malicious actor.  

DDoS attacks have become a frequent hazard, as they are commonly used to take revenge, conduct extortion, activism, and even for cyberwar. In October 2016, leading ISP Dyn's DNS was bombarded by a wave of DNS queries from millions of bots. The attack was executed by the Mirai botnet, and was composed of over 100,000 IoT devices.