More than 1,000 major enterprise networks and small and medium businesses in the U.S. have been compromised by a recently discovered malware package called "Backoff" and are probably unaware of it, the U.S. Department of Homeland Security (DHS) said in a cybersecurity alert on Friday.

Backoff first appeared in October 2013 and is capable of scraping the memory contents of point of sales systems -- industry speak for cash registers and other terminals used at store checkouts -- for data swiped from credit cards, from monitoring the keyboard and logging keystrokes, from communicating with a remote server.

"Over the past year, the Secret Service has responded to network intrusions at numerous businesses throughout the United States that have been impacted by the "Backoff" malware," the alert said. "Seven PoS system providers/vendors have confirmed that they have had multiple clients affected."

The malware is thought to be responsible for the recent data breaches at Target, SuperValu supermarkets and UPS stores, and the Secret Service is still learning of new infections.

DHS first warned of Backoff in late July, when it noted the malware was not detectable my most antivirus software. That made it particularly difficult to stop, because much of the fight against computer viruses and malware rests on antivirus applications.

Most antivirus packages now detect Backoff, but DHS is advising network operators take immediate action to ensure they haven't been affected.

"DHS strongly recommends actively contacting your IT team, antivirus vendor, managed service provider, and/or point of sale system vendor to assess whether your assets may be vulnerable and/or compromised," it said. "The Secret Service is active in contacting impacted businesses, as they are identified, and continues to work with and support those businesses that have been impacted by this PoS malware."

In many cases, hackers gained access to machines through brute-force attacks on remote log-in systems offered through companies like Microsoft, Apple and Google and other third-party vendors. Once inside, they were able to copy the malware to the machine and set it capturing credit card data.

The DHS asked that instances of it are reported to a local Secret Service field office.

The Target data breach was one of the largest in recent memory, resulting in tens of millions of credit and debit cards being compromised. In the last couple of weeks, SuperValu said that at least 180 of its stores had been hit by a data breach and earlier this week UPS said 51 of it UPS Store locations had been hit.

Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com