The Information Commissioner's Office (ICO) has fined Cathay Pacific Airways £500,000 for failing to protect customers' personal data.
The UK watchdog said the airline's computer systems had exposed details of 111,578 UK residents and a further 9.4 million people from other countries.
These included names, passport details, dates of birth, phone numbers, addresses and travel history.
"Appropriate security" was not in place between October 2014 and May 2018.
The ICO said Cathay Pacific became aware of a problem in March 2018, when it suffered a "brute force" password-guessing attack.
The Hong Kong-based firm reported this to the ICO. The regulator said it subsequently uncovered "a catalogue of errors" during a follow-up investigation, including:
- back-up files that were not password protected
- internet-facing servers without the latest patches
- operating systems that were no longer supported by the developer
- inadequate anti-virus protection
At least one attack involved a server with a known vulnerability - but the fix was never applied, despite having been public knowledge for more than 10 years.
Steve Eckersley, the ICO's director of investigations, said there were "a number of basic security inadequacies across Cathay Pacific's system, which gave easy access to the hackers".
The airline failed four out of five of the basic cyber-essentials guidance from the National Cyber Security Centre, he added.
Analysis: A wake-up call for others
By Joe Tidy, Cyber-security reporter
I'm told investigators were extremely concerned by the failures they found. It paints a picture of a company that did not take security of personal data seriously, and today's fine will be a wake-up call to them and other firms. It is, however, only a pittance compared to what it could have been if the hack had occurred more recently.
New GDPR rules have increased the potential maximum fine, and it's clear the failures here would have warranted a far more severe punishment.
Instead of a £500k penalty, Cathay Pacific could have been hit with a share-holder sickening £470m fine - 4% of its annual global turnover.
The £500,000 fine Cathay Pacific is facing is the maximum possible under the Data Protection Act 1998, which was used instead of the newer GDPR "due to the timing of the incidents in this investigation".
In July 2019, the ICO announced it would fine British Airways £183m for a breach of its systems, and the Marriott hotel group £99.2m. But both fines were delayed until later this year.
The ICO said that Cathay Pacific had acted promptly once it became aware, and sought expert help from a top cyber-security firm, and had also contacted affected customers.
The report also noted there were no confirmed cases of the personal data being misused - but that it was very likely it would be in future.
In a statement about the fine, Cathay Pacific said it "would once again like to express its regret, and to sincerely apologise for this incident".
It said "substantial amounts" of money had been spent on security in the past three years.
"However, we are aware that in today's world, as the sophistication of cyber-attackers continues to increase, we need to and will continue to invest in and evolve our IT security systems."