A data breach at department store chains Saks Fifth Avenue, Saks Off Fifth and Lord & Taylor has compromised the personal information of customers.

The chains’ parent company, Canada-based Hudson’s Bay, announced the breach of its store payment systems on Sunday. The company said it was investigating and taking steps to contain the attack.

The disclosure came after New York-based security firm Gemini Advisory revealed on Sunday that a hacking group known as JokerStash or Fin7 began trying to sell a stash of up to 5m stolen credit and debit cards on dark websites last week.

The security firm confirmed with several banks that many of the compromised records came from Saks and Lord & Taylor customers.

Hudson’s Bay said in a statement that it “deeply regrets any inconvenience or concern this may cause”. It has not said how many Saks or Lord & Taylor stores or customers were affected.

There is evidence that the breach began about a year ago, said Dmitry Chorine, Gemini Advisory’s co-founder and chief technology officer. The prolific hacking group has previously targeted major hotel and restaurant chains, he said.

Chorine said the hackers’ method was to send cleverly crafted phishing emails to company employees, especially managers, supervisors and other key decision-makers. Once an employee clicks on an attachment, which is often made to look like an invoice, the system gets infected.

“For an entire year, criminals were able to sit on the network of Lord & Taylor and Saks and steal data,” he said.

Chorine said most of the stolen credit cards appear to have been obtained from stores in the New York City metropolitan area and north-eastern US states. It was possible, he said, that those stores had not yet adopted the more secure credit card payment systems that have been rolled out elsewhere.

Hudson’s Bay said there was no indication that the breach affected its online shopping websites or other brands, including Home Outfitters and Hudson’s Bay stores in Canada.

The company said customers would not be liable for fraudulent charges. It plans to offer free credit monitoring and other identity protection services.