The UPS Store said Wednesday that malicious software was found on the systems of 51 of its franchises in 24 U.S. states, although no fraud has been detected yet.

The subsidiary of United Parcel Service, which has 4,470 U.S. stores, said in a notice that customer names, postal and email addresses and payment card information may have been exposed.

The earliest evidence of malware found on a system dates to Jan. 20, The UPS Store said. Most of the 51 stores were affected between March 26 and Aug. 11, when the malware was removed. The UPS Store said it is now safe to use payment cards at its franchises.

“We know it is our responsibility to protect customer information, and we have taken a number of steps to ensure that customers can remain confident about doing business with The UPS Store centers network,” it said.

Credit monitoring and identity theft services will be provided to customers for one year. An internal review is underway, and The UPS Store said it has retained an IT security firm.

The UPS Store said it did not have enough information to contact affected customers directly. Its public disclosure is intended to help customers who used a credit or debit card at one of the affected stores to know if they are at risk.

Numerous retailers, including Target and Neiman Marcus, have disclosed data breaches over the past year that were attributed to malware targeting point-of-sale systems, the computerized cash registers that process payment cards.

The attacks have proved highly successful at capturing payment card details, which are then sold in underground online markets.

Supervalu, one of the largest grocery wholesalers and retailers in the U.S., said last week that 180 of its stores were affected by a payment system intrusion between June 22 and July 17. The company runs stores under brands including Hornbacher’s, Shop ‘n Save, Shoppers Food & Pharmacy, Farm Fresh and Cub Foods banners.