The Yomiuri Giants said Thursday that a cyberattack on the system managing official fan club member data leaked 807,030 email addresses, with the breach running from around 3 a.m. to about 9 a.m. that day.
The team said it holds no credit card information, and that card data was not among what leaked.
The Giants announced the leak on Thursday, the same day the attack unfolded. Details of how the attackers got in, and who was behind it, did not accompany the announcement.
The disclosure gives members the number of addresses exposed and the hours the breach ran, but not how the intrusion was detected or when those affected were told.
Fan data as a soft target
Fan club systems collect names, addresses and email addresses from supporters who sign up for newsletters, tickets and members-only content. Such databases are attractive to attackers because they are large and often run separately from a club's core business systems.
The Giants are one of Japan's most heavily supported professional baseball teams, and their fan club reaches a national membership beyond Tokyo.