A phishing attack may have resulted in the theft of personal information belonging to customers of Japan Airlines's frequent flier club.

The data compromised includes names, addresses, genders and places of work of anywhere between 110,000 and 750,000 members of the program, according to the Japan Times.

Following an investigation – which found that 23 computers contained malware – the airline determined that no credit card or financial information was impacted by the breach. The airline detected the intrusion on Friday and Monday, however, it believes the attacks have gone undetected for more than one month and were introduced to the airline's network via a phishing email.

This incident follows a similar attack on the airline in February, in which hackers penetrated a different program Japan Airlines offers, which allows customers to trade in mileage points for gift coupons.