Shoppers who used credit cards or debit cards at Target between November 27 and December 15, 2013 may have been hacked. The Secret Service is investigating the breach.

The breach first came to light via a report from security researcher Brian Krebs that said Target had suffered a data breach around the time of Black Friday last month "potentially involving millions of customer credit and debit card records."

Target said it notified authorities and financial institutions immediately after it was made aware of the unauthorized access, and had hired a forensics team to thoroughly investigate how the breach may have happened, said CNN.

The thieves reportedly gained access to data on the magnetic strips of shoppers' cards, potentially allowing them to produce counterfeit versions, according to CNN.

The thieves could also potentially withdraw cash from ATMs using counterfeit debit cards if they were able to intercept PIN data from Target, CNN said.