DNSSEC, a security measure now being deployed by many registrars to guard against DNS tampering may not have prevented this kind of attack, said Paul Mutton , a security analyst with Netcraft.

DNSSEC uses public key cryptography to digitally "sign" the DNS records for websites. It is designed to stop attacks such as cache poisoning, where a DNS server is hacked, making it possible for a user to type in the correct website name but be directed to a fake website.

"If the attacker was able to change the DNS settings held by the domain registrar, presumably they could also have changed other settings, such as disabling DNSSEC, or rather, simply change the DNS settings to point to nameservers that do not support DNSSEC."

NetNames described the attacks against its systems as being "sustained and concentrated." "We will continue to review our systems to ensure that we provide our customers a solid, robust and above all secure service," it said.