Bank of Montreal says fraudsters may have accessed some customers personal and financial information.
The bank didn't say how many clients may have been affected, only describing the number as, "limited."
It says it is contacting those who may have been affected. The bank also recommends customers monitor their accounts for anything suspicious.
Word the potential breach came shortly after Simplii Financial made a similar announcement.
It said approximately 40,000 clients may be affected
Simplii, which is the Canadian Imperial Bank of Commerce's direct banking brand, said it learned of the potential issue on Sunday and has implemented additional online security measures as it continues to investigate.
It adds that clients who are victims of fraud because of the issue will receive 100 per cent of the money lost from the affected bank account.
Simplii said there is no indication that clients who bank through CIBC have been affected.
With files from the Canadian Press