Company officials said the cyberattack could be the worst breach in South Korea since hackers hit the Web site of Internet Auction Co., a South Korean unit of eBay Inc., affected 18 million users in 2008, Yonhap reported.
Koo Ki-hyang, a spokeswoman for SK Communications, said officials discovered Tuesday the company's Nate search engine and Cyworld social networking Web site were infected by a malicious code. Koo said the code apparently originated from an address in China, adding that the company couldn't identify when the information was hacked or who was responsible.
SK Communications, an affiliate of SK Group, asked police to investigate the incident.
"Police will first determine whether the hacking was conducted internally or by an outsider," an official at the Cyber Terror Response Center said. "We will also look into whether the customer information leak was triggered by malicious Internet Protocol codes from China."
Yonhap said Nate.com is the third-most visited Web search engine in South Korea and Cyworld has 25 million users.