A devastating attack that exposed the personal information of 35 million South Koreans was perpetrated after hackers breached the security of popular software provider ESTsoft and planted malicious code on one of its update servers, it was widely reported Thursday.

Attackers with Chinese IP addresses uploaded malware to a server used to update ESTsoft's ALZip compression application, South Korean news outlets said. The upgrades eventually caused the compromise of 62 PCs at SK Communications that used the program. Attackers then tapped the machines to steal the names, user IDs, hashed passwords, birthdates, genders, telephone numbers, and street and email addresses contained in a database connected to the same network.

It was South Korea's biggest theft of personal information ever. With about 49 million people living in South Korea, the breach is believed to have affected the majority of the nation's population.

After hijacking the SK Communication PCs with the fake ALZip update, the attackers used the machines to access databases containing user information for the telecom's Cyworld social networking website and the Nate web portal. The publications cited investigators from Korea's National Police Agency.

“As a general-purpose software company, we deeply apologize for being involved in the hacking,” ESTsoft CEO Kim Jang-joon said in a statement, according to The Korea Joongang Daily. “We respect the results of the police's analysis and investigation. To prevent further hackings, we will strengthen the security system of our programs.”

Kim said other software titles offered by ESTsoft, including its Alyac antivirus application, weren't affected in the breach.

Koreans have been dumbstruck at the news. NHN, the operator of Korea’s No. 1 web portal, ordered its employees to delete ESTsoft programs, The Korea Joongang Daily reported in an earlier article. At least one lawsuit has already been filed against SK Communications.

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