The cyber attacks that put 40 million credit card numbers and other information stored in Target Corp. cash register systems at risk could be even more widespread than previously thought. According to an advisory released Friday, The U.S. Department of Homeland Security and U.S. Secret Service warned that retail companies should check their in-store Point-of-Sale systems for the malware called Backoff, which could affect more than 1,000 U.S. businesses.

In addition to Target, the United Parcel Service of North America, Inc. (UPS) and Supervalu, Inc. also confirmed they were subjected to security breaches. While no other companies have stepped forward with similar confirmations, the Secret Service and DHS warned there could be more. The Backoff malware affects Point-of-Sale (POS) systems from at least seven vendors, and the advisory asks all companies with POS systems to check for possible intrusions.

Cyber criminals using the malware could steal data stored in the POS systems, which might include credit card numbers, mailing addresses, phone numbers and other personal information. The systems at Target were exploited for weeks before anyone detected a breach. Therefore, the advisory warns that stores may not even be aware their systems have been compromised and that they should check for the malware as soon as possible.