Target security breach: 40 million accounts possibly affected

Target confirmed Thursday that a security breach has possibly affected about 40 million credit and debit card accounts. Customers who shopped at the chain between Nov. 27 and Dec. 15 may have been exposed, Target says.

Target says it immediately notified authorities and financial institutions when it became aware of the breach. The chain has a third-party firm investigating the incident. The Secret Service confirmed to ABC News on Wednesday they are investigating the massive breach.

According to The Wall Street Journal, the breach involves the theft of data stored on the magnetic stripe of cards used at store registers across the United States. Officials say the machines may have been tampered with. The Wall Street Journal reports customers who shopped online do not appear to be affected.

Target is advising customers who suspect any unauthorized activity on their cards to call the company at 866-852-8680.

The discount chain has 1,797 stores in the United States.