Bank of Montreal (BMO) and Simplii Financial, a digital bank owned by Cibc, are both warning customers that crooks are claiming to have stolen their personal data.
In a statement, BMO says that fraudsters contacted the bank on Sunday claiming to be in "possession of certain personal and financial information for a limited number of customers".
Simplii has been more specific on numbers, reporting that the crooks appear to have details on more than 40,000 of its customers.
BMO says that it believes that the hack originated from outside of Canada and that it is "confident that exposures identified related to customer data have been closed off".
Both banks are advising customers to notify them of any suspicious account activity, with Simplii promising to reimburse anyone who loses money.