Were you one of the tens of millions of customers that shopped at your local Target store over the Black Friday shopping weekend? Well if you were, you may want to read this and then immediately check your online banking.

Reuters reported late Wednesday evening that “track data” from at least 1 million payment cards was thought to have been stolen. The thieves were able to obtain this information via security installed on the machines customers use to swipe their magnetic card data for purchases. It’s believe that all 1,797 stores in the United States have been breached.

“When all is said and done, this one will put its mark up there with some of the largest retail breaches to date,” Krebs on Security said citing an unnamed source.  The Secret Service confirmed to USA Today that they are investigating the matter:

“The Secret Service will confirm it is investigating the incident at Target,” spokesman Brian Leary said in a telephone interview Wednesday night. “We don’t have any further comment because it’s an ongoing investigation.”

To date the largest retail credit card security breach was an attack on TJX Cos the parent company of TJ Maxx, Marshalls and Homegoods. That data theft in March of 2007 was initially reported as a theft of over 47.5 million payment cards over a period of 18 months. It’s believed that number could be as high as 94 million account numbers. The attack at Target is said to be bigger.

Reuters reports that the attack at Target began Black Friday and could have run as long as December 15th, the busiest time of the year for the Minneapolis based retailer. Reuters says that American Express has responded saying they are putting fraud controls in place. They had no response form Visa or Mastercard.

Mastercard Vice President of Communications, James Issokon told USA Today  “at this point is best directed to Target.”  The breach affected all brands of credit cards used for payment at physical Target locations. This reported breach did not affect customers using target.com