The airline said it had identified unauthorised access to its systems in March. The EU's General Data Protection Regulation requires notification of data breaches within 72 hours. Cathay Pacific has operations in Europe as well.
In a statement the airline said the data that had been accessed was passenger name; nationality; date of birth; phone number; email; address; passport number; identity card number; frequent flyer programme membership number; customer service remarks; and historical travel information.
Apart from this, 403 expired credit card numbers were accessed and 27 credit card numbers with no CVV were accessed.
The data theft was later revealed to have been carried out by a group known as Magecart which used scripts to redirect data to them from the airline's payment forms.
Randy Abrams, senior security analyst at security firm Webroot, said apart from the hit to its reputation, Cathay Pacific "may face costly GDPR repercussions due to the amount of time that passed between the discovery of the breach and reporting it to the public".
Cathay Pacific chief executive Rupert Hogg said, “We are very sorry for any concern this data security event may cause our passengers.
"We acted immediately to contain the event, commence a thorough investigation with the assistance of a leading cyber security firm, and to further strengthen our IT security measures.
“We are in the process of contacting affected passengers, using multiple communications channels, and providing them with information on steps they can take to protect themselves.
"We have no evidence that any personal data has been misused. No-one’s travel or loyalty profile was accessed in full, and no passwords were compromised.”
Abrams added: "Airlines appear to be an increasingly popular target for cyber criminals. In recent months, Air Canada and British Airways have suffered breaches.
"However the Cathay Pacific breach disclosed a feature-rich set of data, including more than 40 times more passports than the Air Canada breach, meaning it will have a much greater impact on passengers.
"It is clear that all airlines are squarely in the crosshairs of cyber criminals, and as such must be significantly more vigilant. Airlines are not random targets to cyber criminals.”
He said in addition to likely theft of money, the high number of passports which had been compromised should cause concern to governments as they tried to secure their borders.
"The sheer amount and quality of data leaked can make for extremely targeted social engineering attacks. Being able to incorporate details such as travel history can enable cyber criminals to create exceptionally plausible social engineering attacks against enterprises, helping fuel future attacks," Abrams added.