A recent attack on Japan's national pension system compromised the information - including names, pension identification numbers, addresses and birth dates - of more than 1.25 million people, according to a report in the Wall Street Journal.
The breach, discovered on May 28, was the result of pension employees opening a malicious email attachment.
Authorities said the pension fund's core system, which houses the most sensitive data including payment and benefit information, wasn't affected. Pension system President Toichiro Mizushima promised new pension identification numbers to those affected, the Journal said.
This is the second major incident the public pension system has experienced in recent times. In 2007, millions of premium payments went unaccounted for. That scandal contributed to the defeat of the ruling Liberal Democratic Party 2007 elections. After this latest incident, officials said an outside committee will investigate and help strengthen security.
