Silo focuses mainly on leveraging Oracle to obtain a shell and escalate privileges. It was intended to be completed manually using various tools, however Oracle Database Attack Tool greatly simplifies the process, reducing the difficulty of the machine substantially.

Bastard is not overly challenging, however it requires some knowledge of PHP in order to modify and use the proof of concept required for initial entry. This machine demonstrates the potential severity of vulnerabilities in content management systems.

Optimum is a beginner-level machine which mainly focuses on enumeration of services with known exploits. Both exploits are easy to obtain and have associated Metasploit modules, making this machine fairly simple to complete.

Devel, while relatively simple, demonstrates the security risks associated with some default program configurations. It is a beginner-level machine which can be completed using publicly available exploits.

Blue, while possibly the most simple machine on Hack The Box, demonstrates the severity of the EternalBlue exploit, which has been used in multiple large-scale ransomware and crypto-mining attacks since it was leaked publicly.

Legacy is a fairly straightforward beginner-level machine which demonstrates the potential security risks of SMB on Windows. Only one publicly available exploit is required to obtain administrator access.

FriendZone is an easy difficulty Linux box which needs fair amount enumeration. By doing a zone transfer vhosts are discovered. There are open shares on samba which provides credentials for an admin panel. From there, an LFI is found which is leveraged to get RCE. A cron is found running which uses a writable module, making it vulnerable to hijacking.

Sunday is a fairly simple machine, however it uses fairly old software and can be a bit unpredictable at times. It mainly focuses on exploiting the Finger service as well as the use of weak credentials.