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New JavaScript sniffer is similar to malware used in the Magecart campaign last year that affected over 800 sites.

Criminals using just one line of malicious code have successfully compromised at least seven e-commerce sites and potentially stolen payment card data belonging to thousands of customers of the online stores.

Six of the e-commerce sites are US-based and one, belonging to footwear maker Fila, is based in the United Kingdom. Security vendor Group-IB, which uncovered the attacks, identified the malicious code as a new JavaScript sniffer (JS sniffer) that it has named GMO.

In an alert Thursday, the Moscow-based Group-IB said its researchers first discovered the GMO JS sniffer on Fila UK's website in early March. The company's subsequent investigations showed that the malware has been active since at least May 2018 and that six relatively small US e-commerce sites have been affected as well.

Group-IB identified the six sites as forshaw.com, absolutenewyork.com, cajungrocer.com, getrxd.com, jungleeny.com, and sharbor.com. The security vendor pegged the combined total number of monthly visitors to these sites at around 350,000.

Group-IB described GMO as a family of JS sniffers that targets e-commerce sites based on the Magento content management system. The malware is one of at least 38 families of JS sniffers designed to steal payment card data and credentials from online stores. The attackers are injecting the card-stealing code into the sites either via an unknown vulnerability in Magento or by somehow compromising the credentials on the website administrator, Group-IB said.

Once in place, the one-line code downloads the JS sniffer whenever a user lands on the compromised e-commerce website's checkout page. The sniffer then intercepts any credit card data that is entered into the page, puts the data it into local storage, and then sends it out to a system controlled by the attacker.