Around 20,000 users of the Air Canada App may have had their personal information 'improperly accessed.'
In a notice to customers, company officials say unusual login behaviour was discovered on the mobile app between August 22 - 24.
All 1.7 million accounts have been locked until users change their passwords as a precaution.
Officials say credit card information is protected, but encourage users to always monitor transactions for anything suspicious.
Customers who stored passport information on their profile are told the risk of a third party obtaining a passport in your name is low if you still have your passport, proof of citizenship, and supporting identity documents; a new passport cannot be issued to anyone solely based on the information found in a passport.