The "Backoff" point-of-sale malware isn't showing any signs of backing off.

The Secret Service now puts that number of infected systems at more than 1,000 U.S. businesses. Backoff was first identified in October 2013 and has turned up in three Secret Service investigations.

Once installed, the malware scrapes PoS system's memory for "track data," installs keyloggers, returns findings to a centralized command server and installs a supplemental backdoor.

A high rate of return is possible for the hackers as PoS systems set up on a local network are easy targets for the attackers. They can gain access and remain hidden for long periods of time without being caught.

Network security firm Damballa detected a 57 percent rise in infected devices in August, and that number grew another 27 percent last month.