That finding comes from security firm Trend Micro, which has reported seeing a spike in attempted Zeus Trojan application infections beginning in February 2013 and peaking in May. Zeus malware targets personal and financial data stored on Windows PCs and is controlled via a "Zbot" botnet.
"Old threats like Zbot can always make a comeback because cybercriminals profit from these," said Jay Yaneza, senior technical manager at Trend Micro, in a blog post. "Peddling stolen banking and other personal information from users is a lucrative business in the underground market. Plus, these crooks can use your login credentials to initiate transactions in your account without your consent."
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Zeus also can press infected PCs into service as nodes in a botnet composed of similar "zombie" PCs. Such botnets might comprise hundreds or thousands of systems and be tapped by attackers -- or rented out -- to serve as spam email relays or malware attack launch pads, or to generate distributed denial-of-service (DDoS) attacks.
Not all Zeus infections stem from spam emails. Criminal gangs also regularly post links to malicious websites that launch drive-by attacks that result in Zeus installations. Recent attack campaigns have involved links on supposed NFL fan pages on Facebook, as well as e-commerce sites selling fake Nike shoes, according to Eric Feinberg, founder of the advocacy group Fans Against Kounterfeit Enterprise (FAKE).
"If you really want to hack someone, the easiest place to start is a fake Facebook profile -- it's so simple, it's stupid," Feinberg told The New York Times.
According to Trend Micro, the recent spike in Zeus activity has largely involved two variants of the malware: Citadel, which first appeared in 2011 and is apparently the brainchild of Russian and Ukrainian programmers who worked with source code published by Zeus' developer; and Gameover, which is designed to steal bank and credit card details and has been distributed via massive spam campaigns.
Zeus first shot to cybercrime fame in 2006, gaining notoriety as king of automated attack toolkits. Subsequent versions of the malware have continued to add features and functionality. The Zitmo variant, for example, was adapted in 2011 to target Android mobile devices and steal the one-time passwords -- known as mobile transaction authentication numbers (mTANs) -- used by many banks.
As of 2010, a basic version of Zeus was fetching $3,000, although add-ons could boost the purchase price to above $10,000. As those prices suggest, Zeus attacks can be lucrative. For example, the Eurograbber campaign, discovered last year, used Zeus malware to steal an estimated $47 million from more than 30,000 corporate and private banking customers across Europe.
Many different, unconnected Zeus botnets are typically running at any given time. The Zeus Tracker project, for example, which counts Zeus command-and-control (C&C) servers, currently reports that it's tracking 800 such servers. But related malware variants used by the attackers are detected by antivirus software only about 38% of the time. That low detection rate is typically due to the malware being polymorphic, meaning that the attack code is regularly repackaged so that it remains functionally equivalent but doesn't match known-file signatures.