Saks Fifth Avenue and Lord & Taylor, two high-end US department stores, acknowledge the breach of sensitive information.

A well-known ring of cybercriminals has obtained more than five million credit and debit card numbers from customers of Saks Fifth Avenue and Lord & Taylor, two high-end US department stores, which are both owned by the Canadian Hudson’s Bay Company.

Gemini Advisory, a cybersecurity research firm that specialises in tracking stolen financial data, disclosed the data breach on Sunday, adding that the information was likely collected at store locations and that online shoppers were probably not affected.

In a blog post, Gemini said that a group of hackers known as Fin7 announced the release for sale of over five million stolen credit and debit cards on JokerStash, an online hub where stolen credit card information is regularly posted.

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JokerStash has previously been used for other high-profile breaches, including Whole Foods, Chipotle, Omni Hotels & Resorts and Trump Hotels.

According to Gemini, preliminary analysis suggests that the criminals siphoned the information from the stores from May 2017 to the present.

The company also said the entire Lord & Taylor network and 83 Saks Fifth Avenue locations had been compromised.

The majority of stolen credit cards were obtained from New York and New Jersey.

Only a limited part of the five million records has been offered for sale so far, approximately 35,000 records from Saks Fifth Avenue and 90,000 records from Lord & Taylor.

The data appears to have been stolen using software implanted into the cash register systems at the affected stores.

It is not clear how exactly the malware was installed in the stores’ checkout systems, Gemini said it was most likely through phishing emails sent to Hudson’s Bay employees.

By opening infected documents, the malicious software was able to spread through the computer system and gather the sensitive information.

Foreign banks at risk

In a statement on its website, Saks Fifth Avenue said the issue had been identified, and steps were being taken to contain it.

“We will offer those impacted free identity protection services, including credit and web monitoring,” Saks Fifth Avenue also said in the statement. Affected customers will “not be liable for fraudulent charges”.

“We are working rapidly with leading data security investigators and also are coordinating with law enforcement authorities and the payment card companies.”

Gemini said in the blog post that, because the targeted stores offer relatively expensive goods, the potential damage to cardholders could be significantly higher in this attack.

It will be difficult for banks to sort fraudulent transactions from those of a legitimate nature, since “cardholders who frequently shop at luxury retail chains like Saks Fifth Avenue are more likely to buy high-ticket items regularly”.

International travellers are also at risk. “We anticipate a significant surge in fraudulent in-person purchases in the coming months, which will explicitly affect foreign banks,” Gemini said.

The Hudson’s Bay Company was founded in 1670 and is one of North America’s oldest department store groups. It has recently appointed a new chief executive in February, Helena Foulkes, and the current data security issue will likely pose a new challenge for her.