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3. Morris worm (1988)
The first malware to be propagated widely via the internet, the Morris Worm or Great Worm hit around 6,000 of the 60,000 computers on the network in 1988.
Robert Morris (pictured), then a student at Cornell University, maintains that his worm wasn't malicious, but was designed to measure the size of the internet. Others took a different view, as the worm used a batch of security flaws in Unix and was launched surreptitiously.
It was certainly unwelcome, as it was far more aggressive at making copies of itself than it needed to be, often infecting machines multiple times, turning what might have been an easily contained annoyance into a powerful denial-of-service attack. Morris became the first person to be convicted under the 1986 US Computer Fraud and Abuse Act, and the incident led to the formation of the first Computer Emergency Response Team Coordination Center, CERT/CC.
Photo credit: Trevor Blackwell/Wikipedia