A data breach that hit Wendy's fast food restaurants was more than three times bigger than originally disclosed and exposed customer credit card data.

The company said Thursday that malware installed in point-of-sale systems was discovered at over 1,000 of its franchised U.S. restaurants -- a big jump from the "fewer than 300 stores" it said in May had been affected.

Hackers gained access to the machines using remote access credentials of a third-party service provider, Wendy’s said.

The breach began in fall 2015 and wasn't discovered until early this year. As part of its investigation, the company discovered a second malware variant had infected its systems.

That second malware targeted "cardholder name, credit or debit card number, expiration date, cardholder verification value, and service code," the company said.

The restaurants affected were all franchise operations and are listed on a website. Wendy's said it's owned and operated restaurants do not appear to have been exposed to the same malware.

The company is one of the latest U.S. brands to have been hit by similar cyber attacks. In recent years, hackers pulled off data breaches against Target and Home Depot, also using login credentials from third-parties.

Hacking attacks against point-of-sale systems, especially at retailers, have become all too common, said Ziv Mador, vice president of security research at Trustwave.

Many of these attacks are conducted by stealing the login credentials used in company web interfaces designed to maintain the point-of-sale systems, he said. Once access is gained, a hacker can easily deliver malware disguised as a security patch.

Retailers tend to reuse the login credentials across their stores, so it can be easy for the hackers to expand their attack, Mador said.

Wendy's is encouraging customers to look out for unauthorized charges on their credit cards.