There isn't one single universally famous cybersecurity quote, but there are a few well-known options depending on the emphasis you want:
Serious
"It's not a question of if, but when".
"Security isn't something you buy, it's something you do" .
Humorous
"Passwords are like underwear: don't let people see it, change it very often, and you shouldn't share it with strangers".
"The most secure computer is the computer that's off".