Marriott International said Friday that up to 500 million hotel guests may have had their data compromised in a hack of the Starwood reservation database.
Marriott said it was alerted on September 8 that there had been an attempt to hack their reservation database in the United States.
The company launched a probe, and discovered "that there had been unauthorized access to the Starwood network since 2014."
They discovered "that an unauthorized party had copied and encrypted information, and took steps towards removing it."
On November 19 Marriott "was able to decrypt the information and determined that the contents were from the Starwood guest reservation database."
Hotels in the Starwood network include Sheraton, Four Points by Sheraton and W Hotels.
"We deeply regret this incident happened," said Arne Sorenson, Marriott's President and Chief Executive Officer. "We fell short of what our guests deserve and what we expect of ourselves."