SK Communications, which runs Nate and Cyworld, said Monday it will start a campaign asking its users to change their passwords to prevent a second possible security breach.

Those who have not changed their passwords since the first security breach, which occurred in late July, must make the change to use the company’s web search engine Nate or social networking site Cyworld, according to its officials.

It also urged them to change their passwords on other sites, such as Naver, Daum, Yahoo!, Paran and Moneta, which users have set up with the same password.

SK Communications plans on running the campaign until all of its members ― 25 million users for Cyworld alone ― have changed their entry codes.

The measure comes after it was discovered that personal information belonging to 35 million online users had been hacked in July, marking the country’s worst security breach.

The users’ data used for log-in included name, birth date, email address, sex, blood type, address and contact information which were all listed in the personal information section of the online communities were taken from a hacker using an Internet Protocol address from China.

This was considered to be a critical issue as Nate is the nation’s third-most visited Web search engine and Cyworld is the largest social networking site with a membership which accounts for half of the South Korean population.

In response, SK Communications released measures including storing less of its customers’ personal information and encrypting more for improved security.

The company also said it will strengthen the role of an organization responsible for security tasks, putting it under the control of the firm’s chief executive officer. It was positioned under the chief financial officer until now.

It will build an online prevention center for the customers so that the company can publicize any phishing methods and findings of spam and malicious codes, said company officials. Company also said it will develop a mobile application aimed at voice phishing prevention.

By Cho Ji-hyun (sharon@heraldcorp.com)