The Gameover Zeus, or GOZ, which was the driving force behind the Cryptolocker malware, may be headed for a resurgence. It was decapitated when the U.S. Department of Justice, law enforcement, foreign government agencies, and private security companies took down its command-and-control structure.

But according to Sean Sullivan, security advisor for F-Secure Labs, compromised machines remain compromised, and the broader threat is still out there. “The botnet was disrupted but not completely destroyed," Sullivan says. "Its creator was not arrested, is still at large, and is currently building a new botnet to replace the old.”

The concern is that a new command-and-control structure could emerge and continue to leverage the millions of compromised systems.

Download the F-Secure Threat Report H1 2014 yourself to dive into the details.