A hacker group that broke into GMO Internet Group services and demanded money posted what appears to be information on users online on the 27th, NHK reported. The company is working to identify those behind the unauthorized access.
The group had threatened to release the data if its demands were not met. GMO has not paid, and the information has now begun to surface.
The group also warned people not to access the company's website, according to NHK, and published the personal information of one individual.
Fallout spreads across finance
The breach has fed a wider wave of concern about leaked personal data. Card fraud linked to stolen information has emerged as a secondary harm, with the latest methods for misuse drawing attention.
Resona, Saitama Resona and Kansai Mirai banks said customer information may also have leaked. A SoftBank subsidiary that suffered a cyberattack disclosed the cause and its countermeasures.
The CEO of Japan Exchange Group said the industry as a whole would work to prevent a repeat of the Iwai Cosmo problem.
For users of GMO's services, the immediate risk is that data already posted cannot be recalled. The company's refusal to pay is a position many security experts endorse, but it does not undo the exposure.