Hackers have attacked South Korea's SK Communications services Cyworld and NATE, accessing the names, phone numbers and email addresses of 35 million users.
"The company has confirmed that a leak of customers' information has taken place due to hacking on July 26," said SK Communications.
"The specific scale of the hacking is still being investigated, but it is estimated that some of the personal information of 35 million Nate and Cyworld members have been leaked."
Joo Hyung-chul, SK Communications CEO, also issued a formal apology.
"Concerning this incident, we offer our apology to our customers and have taken all the necessary measures to minimize the impact and identify the cause and retrieve customer information in cooperation with the authorities."
SK Communications also reported that data like passwords were encrypted.
The Korean Communications Commission revealed the attacks, which affect more than half of South Korea's population, and claims they originated from IP addresses in China. South Korean authorities are currently investigating the breach.
The incident is the latest in a spate of cyber attacks to affect the country. In March 29 different South Korean bodies were affected, including financial institutions, military headquarters and government ministries.
Cyworld is a social network, launched in 1999 and has 25 million users. NATE is a popular South Koren web portal, and the country's third most used search engine.