A member of the Anonymous hacktivist group, who is believed to be Korean, revealed what he claims to be data on customers of the country's No. 5 lender, on his twitter site (@Anonsj).
The hacker who released the data said "the target is KEB (Target: http://www.keb.co.kr/KoreaExchangeBank)."
Released at 7:24 a.m., local time, the documents contain 1,460 e-mail addresses and numbers presumed to be log-in IDs and passwords, according to the hacker. They're mostly Yahoo and Google accounts, with about 50 of them from major Korean portal sites Naver and Daum.
Any evidence to prove that they were Koreans or whether they are KEB clients were not available, according to the documents.
If true, it is the first time a Korean bank has been attacked by the controversial hacking group.
A spokesman from KEB denied the hacker's claim, saying the data does not match with the details of its clients.
"We have a 24-hour security system under constant operation and there was no attempt of a cyber attack," the bank said in a press release.
Anonymous conducted a series of cyber attacks on several North Korean-run Web sites, including Uriminzokkiri, and stole lists of tens of thousands of their subscribers before revealing them to public. The latest attacks came last week on the 101st birthday of the communist regime's founder Kim Il-sung.