More than just a point-of-sale card scraper, it's modular malware, and every module is a rootkit.

Just in time for the holidays, researchers at iSIGHT Partners are warning retailers about ModPOS -- malware in their point-of-sale systems that is nearly impossible to detect, can do a whole lot more than just scrape customers' credit card data, and has already successfully breached U.S. retailers.

"This is by far the most sophisticated PoS malware I've ever seen," says Maria Noboa, senior threat analyst at iSight.

BlackPOS was behind the monster breach at Target, BackOff hit UPS and over a thousand others, and now new PoS malware like CherryPicker and AbaddonPOS have hit the scene. Yet, none of them have the same level of complexity and committment to stealth that ModPOS does.

"It took us two to three weeks just to determine it was malicious," says Noboa. It then took researchers several more weeks to pick ModPOS apart and figure out how it works. In comparison, it only took them about 20 minutes to reverse engineer CherryPicker, says Noboa.

ModPOS is modular. In addition to the PoS card scraper module, it also has a keylogger, an uploader/downloader (with which it could add other pieces), and plug-ins for scraping credentials, and gathering local system and network information.

So, explains Noboa, if it compromises the point-of-sale system and finds that the retailer has wisely encrypted all of the cardholder data or stored it elsewhere, ModPOS can still find ways to make its attack fruitful for them and damaging to the target.

The code was written by someone with exceptional skill, says Noboa. "The shell code they use," she says, "are effectively full programs." They found one piece of shell code contained over 600 functions.

The malware is able to stay persistent and obfuscated because every one of those modules operates in kernel mode, so, "each one is a rootkit," says Noboa.

It's also difficult to detect because all hashes are unique to the victim system. So the researchers can't just hand out a list of hashes along with indicators of compromise, because it wouldn't do anybody any good.

The researchers have not found any evidence of anyone selling ModPOS or even discussing it on underground forums. 

"All of this points to [the malware] being a profit center for someone," says Noboa. They're making their money from use of the malware, not sale of it, and they're not interested in sharing.

All of this has helped ModPOS be quietly lucrative. iSight researchers believe the ModPOS attacks began as early as 2013, and have since stolen millions of credit and debit cards from unnamed U.S. retailers.

They don't know yet how it's being distributed right now or have a fix on the threat actor. They do say there are indications that point to Eastern Europe.

Sara Peters is Senior Editor at Dark Reading and formerly the editor-in-chief of Enterprise Efficiency.