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.