Following a three-month investigation, the company discovered there had been unauthorized access since 2014 and that an "unauthorized party" had copied and encrypted information.
"The company has not finished identifying duplicate information in the database, but believes it contains information on up to approximately 500 million guests who made a reservation at a Starwood property," the company said in a statement.
Marriott said it is working with authorities and has implemented steps to address the breach.
Marriott has launched a website for consumers who are concerned that their information may have been part of the breach and will be notifying customers by email. Access the website here.
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