I was stupid enough to leave the RP2 exposed to the internet with the default password (for the momentary sake of convenience) for a very short time and someone (I am assuming an automated scanning program) changed the password for the pi user and now I can't log in.
Is there a way to undo it?
See How can you recover your password? which explains how to reset the password.
HOWEVER if someone has modified the password there is no way of knowing what else was done.
Either restore from your backup OR do a fresh install - you should have given up on Jessie a year ago - it is unsupported!
The VERY FIRST thing I do on ANY installation is change the password!
