Target is investigating a major credit card breach at its stores that began just after Thanksgiving and ran at least through that weekend and possibly through last Sunday.

Millions of credit cards swiped at stores across the country might be compromised.  Online shoppers appear to be safe. The Secret Service has confirmed that it is investigating what might turn out to be one of the biggest retail breaches to date. The reports suggest that hackers gained access to the data stored on the cards' magnetic stripes.

Worst case, the hackers could "create counterfeit cards by encoding the information onto any card with a magnetic stripe. If the thieves also were able to intercept PIN data for debit transactions, they would theoretically be able to reproduce stolen debit cards and use them to withdraw cash from ATMs."