US supermarket chain Lucky has issued an alert to customers after around 20 branches in the San Francisco Bay Area were compromised with devices that steal credit card information.
The attackers equipped a number of self-service checkouts at the stores with 'skimming' devices that collected customers' credit card data.
The company did not reveal how the checkouts were compromised, but said that it will update security for all self-service systems nationwide as a precaution.
"We are not aware nor have we been notified of any reports that customer accounts were compromised," Lucky said in a notification to customers.
"As a precaution, we recommend customers who used a self check-out lane in the affected stores to verify and monitor all credit/debit accounts with their financial institution to ensure everything is in order."
Skimming devices can be attached to the card-reading hardware on a system, and are most commonly used with cash machines.
Experts have warned that the incident highlights the need for stronger security and better monitoring of transactions by consumers and retailers.
"This again illustrates the sophistication and determination of hackers and their willingness to take risks to steal cardholder data, in this case skimming," said Mark Bower, vice president at Voltage security. "Vigilance and physical security is needed by merchants to prevent it."
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