Some websites running the e-commerce platform Magento appear to have been infected with code that directs victims to the Neutrino exploit kit.
It's not exactly clear how the Magento sites were infected, wrote Denis Sinegubko, a senior malware researcher with Sucuri, a Delware-based security company.
"At this point, we can suspect that it was some vulnerability in Magento or one of the third-party extensions that allowed it to infect thousands of sites within a short time," he wrote.
The Magento sites are rigged to pull content into an iframe from a domain which has been blacklisted by Google, Sinegubko wrote.
According to Google's Safe Browsing service, the malicious domain has been responsible for infecting more than 7,800 websites.
The malicious domain has also been noticed by the security company Malwarebytes in connection with the Neutrino exploit kit, wrote Jerome Segura, a senior security researcher.
Exploit kits are planted by cyberattackers on legitimate web pages. When encountered during normal browsing, the exploit kit probes the computer for software vulnerabilities and if one is present, silently delivers malware.
The malicious domain seen by Sucuri was the one Malwarebytes had seen encountered by web-browsing clients, Segura wrote.
People who encounter the compromised websites with the iframe were bounced through a number of domains, including the malicious one spotted by Sucuri and Malwarebytes.
"The final URL loads the Neutrino exploit kit which fires a Flash exploit and drops the Andromeda/Gamarue malware," Segura wrote. "Compromised machines can be harvested for financial credentials and also become part of a large botnet."
Sinegubko wrote that websites running Magento should be sure that all core files and extensions are updated.
"Since the vulnerability provides access to your database, hackers could use it to create malicious admin users; so it is a good idea to review your site users," he wrote.