Thirty-five million South Koreans – representing a majority of the country's population – may have had their personal information swiped in a single cyberattack.
The attack hit Cyworld, a popular social site similar to Facebook that allows users to share images and updates with friends. The BBC reported that hackers "are believed to have stolen phone numbers, email addresses, names and encrypted information" from the site's millions of account holders.
Also targeted was the NATE Web portal, which gives citizens access to Web services such as email.
SK Telecom, a South Korean mobile telecommunications provider, runs both Cyworld and NATE.
The Korean Communications Commission claimed to have traced the attack back to IP addresses in China, the BBC said.
The massive cyberattack affected roughly 71 percent of South Korea's population of around 49 million.
If Chinese hackers were in fact behind this attack, it would be the latest in a long line of cybercrimes with ties to the country.
In March, hackers thought to be operating in China launched an attack on the European Commission ; two weeks prior, the French Finance ministry suffered a similar fate.
Chinese cybercriminals have also been linked to attacks on Western corporations and government bodies, although in those cases the intruders are believed to be have been working for or with the Chinese government or military.