Postman was an easy (but still tricky) Linux box that made it possible to exploit a publicly accessible Redis service and a vulnerable version of Webmin in order become root.
Monteverde was my fist Windows HTB machine and was very interesting and educative to attack it. This is a very detailed write-up that shows every single step I made to root it.
OpenAdmin was the very first machine of which I made a walkthrough. It was possible to exploit a vulnerable version of OpenNetAdmin application in order to discover juicy information and to get RCE. Then, a misconfigured sudo/vim executable made it possible to privilege escalate.