Thanksgiving Day holiday shoppers line up with television sets on discount at the Target retail store in Chicago, Illinois, November 28, 2013. REUTERS/Jeff Haynes Target was hit by a massive data breach involving millions of customers' credit and debit card information.

The breach impacted customers who shopped in Target's U.S. stores between Nov. 27 and Dec. 15, according to Target.

That window of time includes Black Friday, the busiest shopping day of the year.

The company says as many as 40 million credit and debit card accounts may have been compromised. Information that may have been stolen includes customers' names, card numbers and three-digit security codes.

News of the breach was first reported by journalist Brian Krebs.

"The breach window is definitely expanding," a source identified as an anti-fraud analyst at a top-10 U.S. bank card issuer told Krebs. "We can’t say for sure that all stores were impacted, but we do see customers all over the U.S. that were victimized."

Customers who suspect unauthorized activity are being asked to contact Target at 866-852-8680.