Wendy's executives today released some details of its internal investigation into a hacking incident that was discovered earlier this year when customers began reporting unusual activity on payment cards that were used at some company restaurants.

The fast-food chain reported in the company's first quarter 2016 financial earnings report that malware had been installed in the point-of-sale system (POS) at about 300 of the company's 5,500 locations. The inquiry also uncovered additional information-security issues unrelated to the POS malware at about 50 additional restaurants that is now being corrected.

Wendy's said the POS malware was installed through a third-party vendor.

The POS problem was uncovered in late January when customers began finding unauthorized charges on their payment cards. The company did not say how many customers were affected, but a class action suit against the company has been filed.