Air Canada said it has locked all of its 1.7 million user accounts on its mobile app after the airline discovered that about 20,000 customers’ personal information may have been “improperly accessed.”

In a notice on Air Canada’s website, the country’s largest air carrier said it detected unusual login behaviour on its app between Aug. 22 to 24. The company said it is contacting potentially affected customers directly.

“We immediately took action to block these attempts and implemented additional protocols to block further repeated unauthorized attempts,” Air Canada said on its website. “As an additional security precaution, we have locked all Air Canada mobile app accounts to protect our customers’ data.”

Air Canada added that credit card data is encrypted and would be protected from a breach.

However, users data including Aeroplan numbers, passport numbers, birth dates, nationalities and countries of residence could have been accessed if users saved them in their account profile, the company said.

Air Canada is requiring all of its customers to reactivate the locked accounts by resetting their passwords.

The procedure has prompted a number of complaints on Twitter, with customers citing issues as they tried to change their passwords: