The attack apparently took place in November and ICANN officials discovered it earlier this month. The intrusion started with a spear phishing campaign that targeted ICANN staffers and the email credentials of several staff members were compromised. The attackers then were able to gain access to the Centralized Zone Data System, the system that allows people to manage zone files. The zone files contain quite bit of valuable information, including domain names, the name server names associated with those domains and the IP addresses for the name servers.
ICANN officials said they are notifying any users whose zone data might have been compromised."