Nineveh is not overly challenging, however several exploits must be chained to gain initial access. Several uncommon services are running on the machine and some research is required to enumerate them

CronOS focuses mainly on different vectors for enumeration and also emphasises the risks associated with adding world-writable files to the root crontab. This machine also includes an introductory-level SQL injection vulnerability.

Nibbles is a fairly simple machine, however with the inclusion of a login blacklist, it is a fair bit more challenging to find valid credentials. Luckily, a username can be enumerated and guessing the correct password does not take long for most.

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.