"Although unlikely, it is possible that the passwords of a small percentage of PlayStation Network users may have been changed through unauthorized access, making it possible to view users' personal information and/or use the Wallet for the PlayStation Store."
Sony said that while unauthorized users may be able get past security measures, the PlayStation Store does not display full credit card numbers.
Sony added that it has taken "immediate measures" to fix the security issue.
"We have investigated the extent of unauthorized access and possible alteration of passwords that could have occurred before corrective measures were taken, and are directly contacting customers who may have been affected by this incident."
Sony suggested that PlayStation Network users try signing into the service to make sure that their account hasn't been violated.