A major security breach at Target - affecting tens of millions of people. The retailer says about 40 million credit and debit cards may have been compromised. Security expert Brian Krebs says the breach involved data stored on the magnetic strip of cards used in stores, not online and that the breach may extend to nearly all of the more than 17-hundred target retail stores nationwide. "If the bad guys can steal that information on the back of the card," Krebs said, "they can actually create a second copy of it." Target says the company has moved swiftly to address the issue and is working with law enforcement to resolve it. Security experts tell ABC News that they expect it could be well into the New Year before the issue is fully resolved. Security experts are advising shoppers to check ATM and credit card statements.