South Korea has blamed Chinese hackers for stealing data from 35 million accounts on a popular social network.
The attacks were directed at the Cyworld website as well as the Nate web portal, both run by SK Communications.
Hackers are believed to have stolen phone numbers, email addresses, names and encrypted information about the sites' many millions of members.
It follows a series of recent cyber attacks directed at South Korea's government and financial firms.
Details of the breach were revealed by the Korean Communications Commission.
It claimed to have traced the source of the incursion back to computer IP addresses based in China.
Wave of attacks
The Nate portal gives people access to web services such as email while the Cyworld social site lets people share images and updates with friends and allows them to create an avatar that inhabits a small virtual apartment.
Like many other nations, South Korea has suffered a spate of hacking attacks in recent months. One incident in April targeted a government-backed bank.
A month later, data on more than 1.8 million customers was stolen from Hyundai Capital.
Government ministries, the National Assembly, the country's military HQ and networks of US Forces based in Korea were also hit.
Earlier this year, the South Korean government drew up a cyber security plan in an attempt to thwart the attackers.