Marriott says someone breached its Starwood guest reservation database, potentially exposing information on 500-million guests.
The hotel chain discovered the breach last week and reports is taking steps to fix the security breakdown.
The database included information on guests who made reservations at Starwood hotels on or before September 10th of 2018.
Marriott says it's uncovered unauthorized access that's been taking place within its Starwood network since 2014.
The company says credit card numbers and expiration dates of some guests may have been taken.
For about 327-million people, the information exposed includes some combination of name, mailing address, phone number, email address or passport number.
It could also include Starwood Preferred Guest account information, date of birth, gender, arrival and departure information, reservation date and communication preferences.
Marriott has set up a website and a call centre to answer questions.