Credit and debit card accounts stolen in a recent data breach at retail giant Target have been flooding underground black markets in recent weeks, selling in batches of one million cards and going for anywhere from $20 to more than $100 per card, KrebsOnSecurity has learned.

Prior to breaking the story of the Target breach on Wednesday, Dec. 18, I spoke with a fraud analyst at a major bank who said 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.

There are literally hundreds of these shady stores selling stolen credit and debit cards from virtually every bank and country. But this store has earned a special reputation for selling quality “dumps,” data stolen from the magnetic stripe on the backs of credit and debit cards. Armed with that information, thieves can effectively clone the cards and use them in stores. If the dumps are from debit cards and the thieves also have access to the PINs for those cards, they can use the cloned cards at ATMs to pull cash out of the victim’s bank account.

At least two sources at major banks said they’d heard from the credit card companies: More than a million of their cards were thought to have been compromised in the Target breach. One of those institutions noticed that one card shop in particular had recently alerted its loyal customers about a huge new batch of more than a million quality dumps that had been added to the online store. Suspecting that the advertised cache of new dumps were actually stolen in the Target breach, fraud investigators with the bank browsed this card shop’s wares and effectively bought back hundreds of the bank’s own cards.

When the bank examined the common point of purchase among all the dumps it had bought from the shady card shop, it found that all of them had been used in Target stores nationwide between Nov. 27 and Dec. 15. Subsequent buys of new cards added to that same shop returned the same result.

On Dec. 19, Target would confirm that crooks had stolen 40 million debit and credit cards from stores nationwide in a breach that extended from Nov. 27 to Dec. 15. Not long after that announcement, I pinged a source at a small community bank in New England to see whether his institution had been notified by Visa or MasterCard about specific cards that were potentially compromised in the Target breach.

This institution has issued a grand total of more than 120,000 debit and credit cards to its customers, but my source told me the tiny bank had not yet heard anything from the card associations about specific cards that might have been compromised as a result of the Target breach. My source was anxious to determine how many of the bank’s cards were most at risk of being used for fraud, and how many should be proactively canceled and re-issued to customers. The bank wasn’t exactly chomping at the bit to re-issue the cards; that process costs around $3 to $5 per card, but more importantly it didn’t want to unnecessarily re-issue cards at a time when many of its customers would be racing around to buy last-minute Christmas gifts and traveling for the holidays.

On the other hand, this bank had identified nearly 6,000 customer cards — almost 5 percent of all cards issued to customers — that had been used at Target stores nationwide during the breach window described by the retailer.

“Nobody has notified us,” my source said. “Law enforcement hasn’t said anything, our statewide banking associations haven’t sent anything out…nothing. Our senior legal counsel today was asking me if we have positive confirmation from the card associations about affected cards, but so far we haven’t gotten anything.”

When I mentioned that a big bank I’d spoken with had found a 100 percent overlap with the Target breach window after purchasing its available cards off a particular black market card shop called rescator[dot]la, my source at the small bank asked would I be willing to advise his fraud team on how to do the same?

CARD SHOPPING

Ultimately, I agreed to help in exchange for permission to write about the bank’s experience without actually naming the institution. The first step in finding any of the bank’s cards for sale was to browse the card shop’s remarkably efficient and customer-friendly Web site and search for the bank’s “BINs”; the Bank Identification Number is merely the first six digits of a debit or credit card, and each bank has its own unique BIN or multiple BINs.

According to the “base” name for all stolen cards sold at this card shop, the proprietor sells only cards stolen in the Target breach.

A quick search on the card shop for the bank’s BINs revealed nearly 100 of its customers’s cards for sale, a mix of MasterCard dumps ranging in price from $26.60 to $44.80 apiece. As one can imagine, this store doesn’t let customers pay for purchases with credit cards; rather, customers can “add money” to their accounts using a variety of irreversible payment mechanisms, including virtual currencies like Bitcoin, Litecoin, WebMoney and PerfectMoney, as well as the more traditional wire transfers via Western Union and MoneyGram.

With my source’s newly registered account funded via wire transfer to the tune of USD $450, it was time to go shopping. My source wasn’t prepared to buy up all of the available cards that match his institution’s BINs, so he opted to start with a batch of 20 or so of the more recently-issued cards for sale.

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