WASHINGTON — More than 1,000 U.S. retailers could be infected with malicious software lurking in their cash register computers, allowing hackers to steal customer financial data, the Homeland Security Department said Friday.

The government urged businesses of all sizes to scan their point-of-sale systems for software known as “Backoff,” discovered last October. It previously explained in detail how the software operates and how retailers could find and remove it.

Earlier this month, United Parcel Service said it found infected computers in 51 stores. UPS said it was not aware of any fraud that resulted from the infection but said hackers may have taken customers’ names, addresses, email addresses and payment card information.

The company apologized to customers and offered free identity protection and credit monitoring services to those who had shopped in those 51 stores.

Backoff was discovered in October, but according to the Homeland Security Department the software wasn’t flagged by antivirus programs until this month.