Heads up to anyone who has shopped at Target stores since Thanksgiving (possibly up until Dec. 15): Your debit and credit card information may now be in the hands of hackers. The Secret Service has confirmed thieves have stolen an unknown number of Target customers' payment information, which could affect millions who have shopped there. Several newspapers and online sites reported the credit card breach occurred the weekend of Black Friday. The thefts took place in Target stores around the country, and may have involved tampering with the machines customers use to swipe cards when making purchases, the newspaper reported. The credit card security breach affected roughly 40,000 card devices at store registers, which could mean that millions of cardholders are at risk. The thieves gained access to data that is stored on the magnetic stripe on the back of the credit and debit cards, according to the newspaper. Online customers were not affected by the breach. Is it safe to shop at Target now? Be your own judge. Be sure to monitor your accounts and report any suspicious activity right away if you have shopped at Target.