A July 4 Magecart card-skimming attack successfully infiltrated 962 online stores in what researchers are calling the largest 24-hour automated Magecart campaign to date.

Researchers from Sanguine Security Labs who detected the attack reported it via Twitter, and uploaded the JavaScript-based skimmer code to GitHub.

Sanguine Security researcher Willem de Groot told BleepingComputer that the campaign took place within a 24-hour timeframe. "It would be nearly impossible to breach 960+ stores manually in such a short time," he reportedly said, noting that some of the victims were vulnerable against PHP object injection exploits.

A look at the uploaded code reveals that the skimmer was developed to steal paying customers’ credit card data, names, phone numbers and addresses (billing and shipping).

It is not currently clear what stores have been victimized.