Cybercriminals are redoubling efforts to steal payment card details from retailers before new defenses are put in place, according to FireEye.

More than a dozen types of malware were found last year that target point-of-sale systems, the electronic cash registers the process payments at many retailers.

Over the last few years, hackers have successfully breached the systems, targeting weaknesses or software vulnerabilities in order to extract card details to sell on the black market.

As of last October, retailers are liable for fraudulent transactions that are not completed using EMV payment cards, which have a microchip and enhanced security defenses that better shield card data.

Major retailers affected by card breaches in the last few years, including Target, have upgraded their systems. But the cost and long delays in getting new systems certified have delayed the transition, leaving a windows for cybercriminals.

Nart Villeneuve, a senior threat intelligence researcher with FireEye, wrote on Monday that more than a dozen malware families that target POS systems were found last year.

"Criminals appear to be racing to infected POS systems in the United States before U.S. retailers complete this transition," Villeneuve wrote.

In response, card issuers and banks have improved their ability to identify and block potentially fraudulent transactions. But the potential windfall has criminals working overtime.

Villeneuve described a new type of POS malware called Treasurehunt, which steals payment card data from a computer's memory.

"In a typical scenario, Treasurehunt would be implanted on a POS system through the use of previously stolen credentials or through brute forcing common passwords that allow access to poorly secured POS systems," he wrote.

Treasurehunt hasn't been seen widely, which indicates that its creators may be deploying it selectively. A string inside its code indicates it was developed by a group that calls itself Bears Inc.

"Bears Inc. is an actor on an underground cybercrime forum dedicated to credit card fraud," Villeneuve wrote. "Bears Inc. has advertised stolen payment card information for sale."

Another string in the code has a playful message: "Greets to Xylitol and co." Xylitol is the nickname of a well-known malware researcher based in France who writes a popular technical blog.

Hacking POS systems has proved profitable for cybercriminals. It's easy to find so-called "carding" forums where payment card details are priced according to how recently the data was stolen and the potential limit of the card. Cybercriminals have found so much low-hanging fruit that the price for stolen card details has actually fallen.