As part of an investigation, the company discovered there had been unauthorized access since 2014 and that an "unauthorized party" had copied and encrypted information. The breach could potentially be one of the largest in history. 

For about 327 million of the guests, it added, the information includes some combination of a name, mailing address, phone number, email address, passport number, Starwood Preferred Guest account information, date of birth, gender, arrival and departure information, reservation date, and communication preferences.