A detailed look at Russian cybercriminals focused on accessing online banking accounts reveals an effective hidden system for spreading malware through compromised websites.The criminal operation, described in a report released Tuesday by email security company Proofpoint, has infected 500,000 mostly U.S.-based PCs with malware capable of recording transactions and stealing credentials. Nearly 60 percent of the operation involves accounts at five of the largest U.S. banks."They are sophisticated when it comes to financially motivated attackers," Wayne Huang, lead researcher for Proofpoint, said.The criminal group's steps to remain under the radar of security researchers start at underground forums where the hackers purchase lists of administrator logins for WordPress sites.The logins are run through a custom-made tool that separates out the working credentials, which the attackers use to log into sites. The hackers then inject server-side script in building a webshell that acts as a backdoor.The hard-to-find webshell lets the hackers download code that can infect systems visiting the site. Before compromising a PC, the hackers use an automated method to check whether it can be exploited through known vulnerabilities in Java, Flash, PDF and Internet Explorer."The filtering enables attackers to maximize their potential for successfully, and invisibly, running an exploit," the report said.All exploits are run through an automated tool that checks whether they can be detected by 25 of the most widely used antivirus products.Once deployed, if an exploit becomes detectable by more than five of the 55 vendors on VirusTotal, then the tool sends an alert over ICQ to the attackers.The first piece of malware installed is Qbot, a banking Trojan designed to steal login credentials. Qbot connects back to the group's command-and-control servers.The group also downloads a second piece of malware called SocksFabric, which lets the hackers create a paid tunneling service that other attackers can access to steal data.Criminals will subscribe to the service, if the Russian group has compromised PCs in particular companies, government agencies or other organizations with valuable data, Huang said.Criminals behind targeted attacks typically know many of the IP addresses used by the would-be victim. "If I'm targeting Boeing, I know exactly Boeing's IP addresses," Huang said.The Russian group is part of a sophisticated subset of financially motivated attackers, the majority of which take a more shotgun approach to compromising websites, typically through SQL or command injections.Such attack methods are more easily detected, which sometimes leads to takedowns of sizable botnets.The Russian group is not as sophisticated as hackers conducting cyberespionage for nation states, Huang said. However, it exemplifies the effectiveness of Russian criminals who have been targeting online banking accounts for "a very long time."