The massive customer data theft from education service provider Benesse Corp. has highlighted the danger of such data — which included the names, dates of birth, gender and parents' address and telephone numbers of millions of children in this case — spreading quickly and limitlessly among businesses that pay to obtain the information they need for targeted marketing. Along with the efforts by companies to prevent leaks of their customer data, measures need to be taken to close the loopholes that allow private information to easily proliferate.

A 39-year-old systems engineer was arrested July 17 for allegedly stealing data from customers of Benesse's services, including a correspondence education courses for children, while he was working as a temporary staffer at a database management contractor for a Benesse affiliate. Given access to the company's database, he is suspected of downloading and copying onto his smartphone data on more than 20 million customers over nearly a year until June. He has reportedly told police that he sold some of the data to a Tokyo-based firm that deals in name lists for ¥2.5 million.

According to some media reports, the name list trader sold the data to 50 companies, including cram schools, kimono stores and cosmetics companies.