overthewire.org
The wargames offered by the OverTheWire community can help you to learn and practice security concepts in the form of fun-filled games.
picoctf.org
picoCTF gamifies learning hacking with capture-the-flag puzzles created by trusted computer security and privacy experts at Carnegie Mellon University.
hackthebox.com
Hack The Box is the Cyber Performance Center with the mission to provide a human-first platform to create and maintain high-performing cybersecurity individuals and organizations.
cmdchallenge.com
Practice your command line interface
pwnable.kr
You know nothing