The hotel group is sending emails to guests affected by the breach.
Marriott International said Tuesday that names, mailing addresses, loyalty account numbers and other personal information of an estimated 5.2 million guests may've been exposed in a data breach. This is the second major security incident to hit the hotel group in less than two years.
Marriott said it spotted that an "unexpected amount" of guest information may've been accessed at the end of February using the login credentials of two employees at a franchise property. The hotel group said information exposed may include names, addresses, emails, phone numbers and birthdays as well as loyalty account details and information like room preferences.
Marriott said the investigation is ongoing but that it doesn't believe credit card numbers, passport information or driver's license numbers were exposed.