Marriott International said it\u2019s investigating a hack of the guest reservation database at its Starwood unit that may be one of the biggest such breaches in corporate history. Marriott shares slumped 5.6% in pre-market trading. 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. Read Also| Meet 4 Indian-origin women who made it big in America; Forbes lists them among US\u2019 top 50 women in tech Regulators and consumers have been stepping up their action against companies that have suffered security breaches as such attacks have increasingly become more severe. Target last year agreed to pay $18.5 million to settle investigations by dozens of states over a 2013 hack of its database in which the personal information of millions of customers was stolen, while Equifax is facing billion-dollar law suits and a regulatory investigation. \u201cThe breach is so big that the company may face a large fine from the authorities and the market is factoring that in,\u201d said Juan Jose Fernandez Figares, chief analyst at Link Securities in Madrid.