The Minneapolis company 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 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 see suspicious charges on the 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. When asked whether there's a certain time when shoppers know their accounts will no longer be vulnerable, a Target spokeswoman said, "We encourage everyone to be vigilant."
But news of the breach comes at the height of the critical holiday shopping season and threatens to scare away shoppers worried about the safety of their personal data. The November and December period accounts for 20 percent, on average, of total retail industry sales.
The issue is particularly troublesome for Target because it has used its red branded credit and debit cards as a marketing tool to lure shoppers with a 5 percent discount.
The company said during its earnings call in November that as of October the percentage of customers who have the Target branded cards topped 20 percent. This holiday season, Target added other incentives to use its cards. Two days before Thanksgiving, Target.com ran a special review sale with 25 exclusive offers, from electronics to housewares for those who used the branded card.
As a result of these incentives, Target says its continues to see that households who activate a Target-branded card have increased their spending at the store by about 50 percent on average.
"This is how Target is getting more customers in the stores," said Brian Sozzi, CEO and Chief Equities Strategist. "It's telling people to use the card. It's been a big win. If they lose that trust, that person goes to Wal-Mart."
Target is just the latest retailer to be hit with a data breach. 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.
Answers to questions about the Target data breach
By BREE FOWLER
The Associated Press
New York • With less than a week until Christmas, a real-life Grinch has stolen the credit and debit card information of about 40 million Target shoppers.
Target says anyone who made purchases by swiping cards at terminals in its U.S. stores between Nov. 27 and Dec. 15 may have had their accounts exposed.
The stolen information included Target store brand cards and major card brands such as Visa and MasterCard.
The data breach did not affect online purchases, the company said.
Here are some answers to the most common questions about the theft:
Q: I shopped at Target during that time. What should I do?
A: Check your credit card statements carefully. If you see suspicious charges, report the activity to your credit card companies and call Target at 866-852-8680. You can report cases of identity theft to law enforcement or the Federal Trade Commission.
You can get more information about identity theft on the FTC's website at www.consumer.gov/idtheft, or by calling the FTC, at 877-IDTHEFT (438-4338).
Q: How did the breach occur?
A: Target isn't saying how it happened. Industry experts note that companies such as Target spend millions of dollars each year on credit card security, making a theft of this magnitude particularly alarming.
Experts disagree about how the breach might have happened.
Q: How can I protect myself?
A: Like they say, cash is king. You can only lose what you're carrying, though admittedly many people may not feel safe walking around with a wad of bills in their pocket.
Q: Can the bad guys be caught?
A: Ken Stasiak, founder and CEO of Secure State, a Cleveland-based information security firm that investigates data breaches like this one, says that given the sophistication of this attack, there is only about a 5 percent chance that the perpetrators will eventually be caught and prosecuted.
He notes that in cases like this, it's hard to determine where the attack originated and given the large mass of information involved it's not going to be found housed on someone's home computer.
Q: How can future breaches be prevented?
A: Avivah Litan, a security analyst with Gartner Research, says an easy way to prevent fraud would be to eliminate the use of easily cloned magnetic strip cards and upgrade to the kind of microchip technology used in most other parts of the world.