Investigators haven't disclosed the vendor's identity or how the hackers stole the credentials.
Credit information was stolen from 40 million customers, as well as personal data from another 70 million people.
Target says that ever since the company confirmed the breach in December, it has taken extra precautions such as limiting or updating access to some platforms.
The company says it's working with the Secret Service and the Justice Department to track down the hackers.
Customers who see suspicious charges on their statements were advised to report them to their credit card companies and call Target at (866) 852-8680.
The Associated Press contributed to this report.