Computer scientist Robert Tappan Morris created the Morris Worm in the late-1980s. His main goal was to approximate the size of ARPANET, a computer network that served as the foundation of the modern internet. The worm spread by replicating itself on thousands of other machines (which makes Morris Worm a type of internet worm), exploiting existing vulnerabilities such as weak passwords, rsh/rexec and UNIX send mail. Unintentionally, the worm caused a Distributed-Denial-of-Service (DDoS) attack when Morris Worm infected around 60,000 machines.