It looks like Target isn't alone in suffering a major data breach recently.
There may be several other retailers besides Target and Neiman Marcus that were hit by hackers over the holiday season, according to Reuters.
The full extent of a recent round of attacks on US retailers' IT systems, thought to have been launched by hackers in Eastern Europe, is yet to be made known to customers affected by them.
Target last week revealed that hackers had pilfered 70 million of its customers' names, mailing address, phone numbers, email address and payment card data — up from the 40 million it initially reported in mid-December.
Last week, upscale department store Neiman Marcus also confirmed that its customer database had been hacked in mid-December, although the company has not revealed how many customers were affected.
In a statement to security researcher Brian Krebs, Neiman Marcus said it was informed of the breach by its credit card processor in mid-December. It too said customer credit card details may have been compromised.
The timing of the breaches has prompted speculation that the retailers were hit by the same hackers; however, it's not been confirmed the two breaches are linked.
According to Reuters, at least three other well-known US retailers were hacked using similar methods as the attack on Target.
Target has not disclosed how the hackers breached its security systems, although Reuters' sources pointed to a sophisticated class of malware known as RAM scrapers, which are built to steal payment data from point of sale systems.
In an interview with CNBC on Sunday, Target CEO Gregg Steinhafel said the company had established that its POS machines were infected with malware.
"What we do know is that there was malware installed on our point of sale registers. That much we have established. We have removed that malware so that we could provide a safe and secure shopping environment." He added that the investigation into the breach is still ongoing.
While the loss of payment card data in the breaches have prompted calls for a review of affected companies' compliance with Payment Card Industry data security standards (PCI DSS), RAM scrapers are designed to bypass encryption methods that are encouraged by the standards.