Wargames

Teaches Joshua the no-win game of Tic Tac Toe and, in the process, averts global thermonuclear war.

The movie "WarGames" released in 1983 features a high school student who loves hacking, played by Matthew Broderick. The protagonist, who broke into a computer at a game development company, correctly guessed the password for the backdoor of the system from the name of his son, who died by the program developer. That is "Joshua". However, the intrusion destination was not the computer of the game company, but the nuclear war simulator "WOPR" of the North American Aerospace Defense Command (NORAD). The protagonist, who intended to start a game of full-scale nuclear war, is about to start a real World War III.

"Except that I never could get Joshua to learn the most important lesson.
What's that?
Futility. That there's a time when you should just give up." from Wargames
Unlike in the movie, where it's just "joshua", the book version uses "joshua5" (because Falken's son died at age 5). It makes the password slightly harder, but we'd still recommend going with something not so guessable.