I'd repoprt it. Not anonymousely but in person. Go to the boss and show him what's going on. If he has even the vaguest idea of computer technology he will see that you didn't abuse the system but that others do.
If he sees that you are honest, you're sure to be promoted or at least get a reward.
If you don't get rewarded, you should tell the whole story to the press (and make them pay you well - it's a top story!)
By staying anonymous or not telling at all, you'll just make yourself suspicious if someone else reports the security breach. It might backfire wery badly on you.