What is behind mysterious 'attacks' at Iran sites?

The group said it had attacked the major Iranian nuclear site at Natanz some two hours earlier, at 02:00 local time. In the detailed message, it claimed it had blown up a facility and that the Iranian regime would not be able to hide it.
The group said it was composed of dissidents within Iran's military and security forces and that they had been behind numerous attacks that the Iranian authorities had so far concealed from the public.