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https://www.smashthestack.org/main.html

A pretty neat wargaming server that uses outdated server software to practice/display different exploits. Its been ages since I've touched it, but iirc the first few levels show off some shell exploits as well as like some strcpy exploits and a buffer overflow with a nop slide. If you know the basics of using a Unix terminal and have some familiarity with any c language, it's pretty accessible. Even if you dont, it has been around long enough that you could probably find a solutions guide or tutorials somewhere online. Thats the quickest way I can think of to get somebody without much time some legal "hacking" experience