Bell Canada says 22,400 of its small business customers have had their account information compromised by hackers.
Bell says in a release the breach of usernames and passwords occurred when an Ottawa-based third-party supplier had its systems hacked, and the information posted to the internet this weekend.
A hacking group calling itself Nullcrew is claiming credit for the attack, and tweeted a link to the data early Saturday.
Bell says five valid credit card numbers were included in the information dump.
The telecom giant says the affected account passwords have been frozen, and that it is contacting impacted customers and credit card companies.
Bell says its own systems were not hacked, and that its residential, mobility and enterprise customers are not affected.