NUKE IT FROM ORBIT. It was when i was doing an assignment with my roommate, i was compiling something on my pi and ran netstat afterwards for no reason. I had an ssh-connection from china (logged in too). The pi was shutdown ASAP, i salvaged everything i needed from the sd and dd'ed raspbian on the disk again.

Turns out you were able to login via root (i thought i disabled it) with the password i set (root...). I learned from this, now external logins are only allowed via private key and i have fail2ban set up