The assets were programmable logic controllers (PLCs). Which controlled systems like prison doors and centrifuges for separating nuclear material.
The "who" is still debated. Credit (if you use that word) is given to the Equation Group. But that is still hotly debated as there seems to be a political agenda with the attack and many still believe this was nation-state sponsored. I can be easy to leave digital fingerprints behind to make it seem like a known hacking team.
