The U.S. Secret Service estimates that more than 1,000 American businesses have been victimized by the type of point-of-sale system malware that recently affected Target and Supervalu.

The Department of Homeland Security on Friday issued an advisory recommending all companies that employ point-of-sale systems monitor their networks for this malware, noting that many businesses are likely affected but don't yet know it.

The malware, known as "Backoff," is used by cyberthieves to steal payment card information from customers of the infected businesses, and has been used in such attacks dating back at least as far as October 2013.

Tim Erlin, a Minneapolis-based IT risk strategist for Portland, Ore.

(AP Photo/Martin Meissner, File)
(AP Photo/Martin Meissner, File) (Martin Meissner/AP)
, software company TripWire, said the recent rash of data breaches involving American retailers is "more of a series than we've ever seen before."

"One big breach is news, but a list of big breaches is clearly a pattern of some type," Erlin said. "We have some evidence that the same tools have been used in these cases, but that doesn't mean the same burglar perpetrated the crimes."

Since Backoff was first identified last year, the Secret Service has responded to cyberattacks "at numerous businesses throughout the United States" that involved the malware, the DHS advisory said, adding that seven point-of-sale system providers have confirmed that several of their clients have been affected.

In July, the Secret Service and DHS