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Cybersecurity firm Easy Solutions "noticed a 10- to 20-fold increase in the number of high-value stolen cards on black market websites, from nearly every bank and credit union," The New York Times reported. Easy Solutions did not immediately respond to Mashable's request for comment.

Security blogger Brian Krebs, who broke the original story about the Target hack, also reported on Friday that thieves have been selling batches of 1 million cards at "$20 to more than $100 per card."

Krebs reported that a fraud analyst at a major bank told him that "his team had independently confirmed that Target had been breached after buying a huge chunk of the bank’s card accounts from a well-known 'card shop' — an online store advertised in cybercrime forums as a place where thieves can reliably buy stolen credit and debit cards."

Once in possession of the credit cards, thieves can clone the cards and use them in stores. If they get access to a customer's PIN, they could also withdraw money from their account. Target CEO Gregg Steinhafel said in a message to customers that "There is no indication that PIN numbers have been compromised on affected bank issued PIN debit cards or Target debit cards."

Target did not immediately respond to a request for comment on the latest reports.

The company said it would notify all 40 million of its affected customers this weekend. These customers made purchases at Target stores between Nov. 27 and Dec. 15.