Target: 40 million credit cards compromised

A breach of credit and debit card data at discount retailer Target may have affected as many as 40 million shoppers who went to the store in the three weeks after Thanksgiving, the retailer said Thursday.

The Secret Service, which is charged with safeguarding the nation's financial infrastructure and payment systems, is investigating the matter, according to CNNMoney.

Target said cards used at the brick-and-mortar stores between Nov. 27 and Dec. 15 may have been impacted.

The thieves reportedly gained access to data on the magnetic strips of shoppers' cards, potentially allowing them to produce counterfeit versions, according to the report. Thieves could also potentially withdraw cash from ATMs using counterfeit debit cards if they were able to intercept PIN numbers.

American Express and Discover have both said they are investigating the incident and had fraud controls in place. MasterCard referred CNNMoney's questions to Target, and Visa did not respond for comment.