Ever since I stumbled upon exploit exercises website - I wanted to try the challenges. They have three main exploitable VMs: Nebula, Protostar and Fusion. The order represents the suggested progression path.
The welcome page reads:
exploit-exercises.com provides a variety of virtual machines, documentation and challenges that can be used to learn about a variety of computer security issues such as privilege escalation, vulnerability analysis, exploit development, debugging, reverse engineering, and general cyber security issues.
Here, I wrote down some of the findings while exploring the Nebula VM.