GoKawiilCriminal hackers have posted the personal data of nearly nine million people online after breaching three UK airports.
Hackers extorted Manchester Airports Group (MAG) for an undisclosed amount but failed to get their ransom paid.
Customers of Manchester, London Stansted and East Midlands airports are being warned of secondary attacks as the data is now available to other criminals.
"MAG is confident that we have taken effective measures to protect our customers and we have contacted all those affected, including reaching out to all those with upcoming bookings to advise them of additional support," the company said in a statement.
Contact details, vehicle registrations and postcodes were stolen in the breach that appears to have accessed databases containing wi-fi log in and car parking details.
The cyber-crime group - which the BBC is not naming - is offering the entire data set for free to other criminals or scammers through their website.
"Today we are releasing the Manchester Group database," they wrote.
"Half a terabyte, and every byte of it is pure PII [personally identifiable information]."