The attack is troubling not just because of its sheer size, but also the level of detail potentially stolen by the attackers.
The hack affects some 500 million guests - and for about 327 million of them, the data included passport numbers, emails and mailing addresses, Marriott said. Some credit card details may also have been taken.
The Marriott hack may rank only below Yahoo as one of the biggest of personal data, when 3 billion users were exposed to a 2013 security breach.
A Marriott statement indicates the hacking was going on years before the company acquired Starwood in a deal valued at about $13.6 billion that closed in September 2016.
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Read also "What Marriott customers need to know about the data security incident involving the Starwood guest reservation database" and "Marriott Security Breach: The human side of cyber security breaches"