American supermarket chain Schnucks says that around 2.4 million cards may have been compromised by a data breach which saw crooks install malware on its systems.
The St Louis-based retailer says that the credit and debit cards were compromised at 79 of its 100 stores between December and March.
The breach first came to light in mid-March when Schnucks' payment processor warned the grocer that 12 cards had been hit by fraud shortly after being used at stores.
With more fraud reports coming in and point-of-sale device tampering ruled out, Schnucks called in forensic investigation firm Mandiant which found malware designed to capture card numbers.
Having only warned customers that their cards could be compromised at the end of March, Schnucks has now revealed the extent of breach. Up to 2.4 million card numbers and expiration dates have been accessed but no names, addresses or any other identifying information.
Scott Schnuck, CEO, says: "We've worked hard to provide a secure transaction environment for our customers and, today I make a personal pledge to you that we will be relentless in maintaining the security of our payment processing system."