Now, a scare for anyone paying for their holiday gifts with credit or debit cards. Target was the target of a major security breach, with credit card information being stolen from its stores, starting around black friday. Rebecca jarvis has all the details. Reporter: This is serious. Top law enforcement officials are stepping in to resolve one of the largest data breaches of all-time. It started at target stores just before black friday. It went on for weeks. And now, millions of americans have been impacted. This morning, abc news has learned that secret service is investigating a data breach at retail giant, target, involving approximately 40 million customer credit and debit cards. The information, believed to have been lifted from roughly 40,000 devices at store registers between november 27th and december 15th of this year. Terrible timing for those eager holiday shoppers who hoped they would be the ones getting the deal. From the bad guys' perspective, that's the perfect day. The more people are swiping their cards. Reporter: Security expert brian krebs said the data was on the magnetic strip on card, not online. And the breach may extend to all of the 1,700 target retail stores nationwide. The name, the card number, the card expiration date. The bad guys can steal that information off the back of the card, they can create a second copy of it. Reporter: Target commenting this morning, we have moved swiftly to address the issue. And are working with law enforcement to resolve it. But security experts tell abc news that they expect we will be well into the new year before the issue is fully resolved, which, unfortunately, could mean a great deal of hassle and headache over christmas for the 40 million americans who shopped at target between, again, thanksgiving thursday and just this past weekend. As for what you can do, first of all, flag this, with your credit card, with your bank information. Check those atm statements. Check the credit card statements. And monitor your credit report for free, at annualcreditreport.Com. I cannot say how important this is. You need to get to this quickly because the faster you get to it, the faster you go to your bank, the faster you talk to your credit card company, the more likely it is to resolve the issue for you. And you have to take this on your own. And I'm one of the shoppers. You have to be on top of those. Thanks, rebecca.