Nintendo has warned its Club Nintendo Japan users to change their passwords after revealing that the member rewards site was hacked in June.

Nintendo was unaware of the hack until July 2 when a dramatic increase in errors revealed tens of thousands of unauthorized logins. The subsequent investigation revealed up to 23,926 stolen logins and almost 15.5 million access attempts.

The attacks were, as of now, limited to Club Nintendo Japan, the US and UK sites do not seem to be effected. Fortunately, Club Nintendo never held any credit card information from its users though the company says it suspects names, addresses, phone numbers and email addresses have all been taken.

In response, Nintendo is bulking-up its security systems and advises anyone who used the same credentials on other sites - such as email - should change those passwords as a precautionary measure.

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[Via Kotaku]