Hong Kong-based Cathay Pacific Airways says it has discovered unauthorised access to the personal data of 9.4 million passengers.
The company said Thursday it had no evidence the leaked data had been misused.
It said the breach was discovered during "ongoing security processes."
Data stolen included names, nationalities; birth dates, phone numbers, addresses, passport and identity card numbers and expired credit card numbers, among other information.
It said no passwords were compromised.
It was contacting customers to advise them on how to protect themselves.
The news caused the airline's shares to plunge 6.5 per cent in early Hong Kong trading.
By midday Thursday, Cathay Pacific's shares were down 5.1 per cent.
Hong Kong's privacy commissioner, Stephen Kai-yi Wong, expressed "serious concern" over the lapse and urged companies to improve protection personal data.
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