South Korea is blaming Chinese hackers for stealing data from 35 million accounts on two websites: Cyworld, a popular social networking platform, and Nate, a web portal.
The enigmatic hackers reportedly extracted names, phone numbers, email addresses and other encrypted information.
Details of the breach were disclosed by the Korean Communications Commission, which claims to have traced the digital infiltration back to a number of IP addresses in China.
SK Telecom, which owns and operates both websites, has published an official apology, while recommending users change their passwords in the wake of the attack.
According to Sophos security analyst Graham Cluley, Cyworld is a virtual "phenomenon" in South Korea, as it boasts approximately 35 million users.
"[This is quite] a staggering figure when you consider the country only has a total population of 49 million people. Let's just reiterate that. 49 million people live in the country, 35 million of them have had their personal information exposed because of this internet hack," Cluley wrote in a blog post.
"Wow. That's amazing. And not in a good way. The most obvious danger is that users of the websites may be targeted by spam and phishing attacks through no fault of their own. Once again, a social network has done a poor job at protecting its users' information, and it's the public which has to carry the burden."
Of course, South Korea is no stranger to hack and extract operations. A number of recent attacks have targeted a government bank, Hyundai Capital, the National Assembly and SK military headquarters.