Shocker, while fairly simple overall, demonstrates the severity of the renowned Shellshock exploit, which affected millions of public-facing servers.

Brainfuck, while not having any one step that is too difficult, requires many different steps and exploits to complete. A wide range of services, vulnerabilities and techniques are touched on, making this machine a great learning experience for many.

Lame is a beginner level machine, requiring only one exploit to obtain root access. It was the first machine published on Hack The Box and was often the first machine for new users prior to its retirement.