UPDATED: 12/19/2013 2:50 p.m.

CHICAGO (CBS/AP) — Target has warned customers as many as 40 million credit and debit card accounts might have been exposed to identity theft, due to a data breach that began at the start of the Black Friday shopping weekend.

The big box chain said customers who purchased items at its U.S. stores between Nov. 27 and Dec. 15 might have been affected. Thieves managed to steal customer names, credit and debit card numbers, card expiration dates, and perhaps the security codes from the back of cards.


Hackers obtained the data by placing malware on Target’s information systems, allowing them to steal the information as customers swiped their cards.

I was buying toys and gifts for Christmas” using a Visa card, one shopper told CBS 2’s Dorothy Tucker. “I was shocked because your information is out there every and people now have it and they can do whatever with it.”

William Kresse, director of the Center for the Study of Fraud and Corruption at St. Xavier University, said potential victims should regularly check their credit and checking account activity to monitor for unauthorized purchases.

“Even if it’s just a small dollar amount, a lot of times fraudsters will send out even less than a dollar charge on accounts, just to see if they’re valid, see if it gets bounced back or not,” Kresse said.

He said it was ingenious of the hackers to time their scheme to coincide with the start of the Christmas shopping season.

Target said it immediately told authorities and financial institutions once it became aware of the breach, and that it is teaming with a third-party forensics firm to investigate the matter and prevent future breaches. It. said it is putting all “appropriate resources” toward the issue.

Target Corp. advised customers to check their statements carefully. Those who suspect there has been unauthorized activity on their cards should report it to their credit card companies and call Target at 866-852-8680. Cases of identity theft can also be reported to law enforcement or the Federal Trade Commission.

Target didn’t say exactly how the data breach occurred, but said it had since fixed the problem and that credit card holders can continue shopping at its stores.

But news of the breach comes at the height of the holiday shopping season and threatens to scare away shoppers worried about the safety of their personal data.

Target is just the latest retailer to be hit with a data breach problem. TJX Cos., which runs stores such as T.J. Maxx and Marshall’s, had a breach that began in July 2005 that exposed at least 45.7 million credit and debit cards to possible fraud. The breach wasn’t detected until December 2006. In June 2009 TJX agreed to pay $9.75 million in a settlement with multiple states related to the massive data theft but stressed at the time that it firmly believed it did not violate any consumer protection or data security laws.

An even larger hack hit Sony in 2011. It had to rebuild trust among PlayStation Network gamers after hackers compromised personal information including credit card data on more than 100 million user accounts. Sony was criticized for slowness in alerting users to the breach.

“Target’s first priority is preserving the trust of our guests and we have moved swiftly to address this issue, so guests can shop with confidence. We regret any inconvenience this may cause,” Chairman, President and CEO Gregg Steinhafel said in a statement Thursday.

Target has 1,797 U.S. stores and 124 in Canada.

(TM and © Copyright 2013 CBS Radio Inc. and its relevant subsidiaries. CBS Radio and EYE Logo TM and Copyright 2013 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed. The Associated Press contributed to this report.)

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