A breach of Marriott International’s reservation database has potentially exposed the private information of as many as 500 million guests, the company has admitted. The company announced Friday that the breach affected its Starwood guest-reservation database. Its investigation showed that an unauthorized party had copied and encrypted information, and that there had been unauthorized access to the Starwood network since 2014. Some payment-card numbers and expiration dates are believed to have been exposed. “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. We are doing everything we can to support our guests, and using lessons learned to be better moving forward.” Marriott will begin sending emails to affected guests today informing them of the breach.