I am taking a Computer Security class and I am supposed to harden my system really well but I am supposed to leave one "hard backdoor" for the adversary to gain root access.
I have been trying to figure something out for two days now and as I am a total beginner, I still can't figure out what would be a good idea to do this.
I am running Ubuntu 14.04. Is there some backdoor a beginner like me can give the adversary to gain root?
Thanks in advance.