WANK worm
The worm that tore through NASA's DECnet network in October 1989 carrying an anti-nuclear message — often called the first hacktivist attack.
On 16 October 1989, two days before the launch of the Galileo probe with its plutonium power source, VMS machines on SPAN and HEPnet started displaying a banner: WORMS AGAINST NUCLEAR KILLERS, followed by a Midnight Oil lyric — "You talk of times of peace for all, and then prepare for war."
The technique was not sophisticated. The worm looked for accounts whose password matched the username or default accounts nobody had changed, altered passwords and spread onwards over DECnet. A year after the Morris worm, that was enough to show that networks built on trust between researchers could not cope with someone who lacked it. The trail led to Melbourne, but nobody was ever charged.