US grocery chain Safeway has confirmed that registers at several stores in California and Colorado had somehow been fitted with "skimmer" hardware to collect payment card information.

According to a report from Krebs on Security citing investigators involved with the case, registers at two stores in northern California and five stores in Colorado were found to have been fitted with the skimming devices. Safeway believes these were unrelated incidents. The source of the devices is not known.

The Krebs report noted that the Colorado discovery was only made after bank customers had reported unauthorized cash withdrawals and that reports of the activity have been coming in since September of 2015.

Safeway said that no card data had been stolen by either of the two skimmers in California, and that a total of three skimmers had been found in Colorado in November.

"When our store teams find evidence of criminal activity like this, we have been able to pinpoint with surveillance video when the devices were installed and how many transactions were processed," Safeway said in a statement to The Register.

"We immediately followed the proper protocol of contacting law enforcement and the banks that service the few cards that were used on those pin pads."

Safeway is advising customers in Colorado to check their bank statements and report any unauthorized activity.

Cash registers and sales terminals have become favorite targets for criminals looking to collect credit card information. Attackers use skimmers or specialized malware packages to collect and then upload card numbers and data for fraudulent transactions or bulk resale online. ®