Clifford Stoll
The astronomer who in 1986 followed a 75-cent accounting error at Lawrence Berkeley Laboratory to an intruder selling secrets to the KGB.
Asked to work out why two billing systems disagreed, Stoll found an unknown user treating the lab's machines as a springboard into MILNET and military computers. Instead of locking him out, he logged every keystroke to printers and planted fake documents about an invented defence project to keep the intruder online long enough to be traced — an early honeypot. The trail ended with Markus Hess in Hanover, who was passing what he found to the KGB.
His book The Cuckoo's Egg (1989) was one of the first to explain network security to a general audience. In 1995 Stoll wrote, with the same confidence, that online shopping and online newspapers would never catch on — a quote the internet has made sure he never lives down.