Comment by a_e_k

See "The Cuckoo's Egg" [1] by Cliff Stoll (who sometimes posts here [2]). A few related HN threads: [3] [4] [5]. The book describes tracking down a hacker in 1986 who exploited a vulnerability in early Emacs' movemail utility.

[1] https://news.ycombinator.com/item?id=49406713