FBI assistant director Joseph Demarest revealed the statistic while briefing a Senate sub-committee about the agency's current and future anti-cyber crime strategy on Tuesday. He said the news is troubling as the botnets' high infection rate costs the US and global economies billions of dollars.
"The use of botnets is on the rise. Industry experts estimate that botnet attacks have resulted in the overall loss of millions of dollars from financial institutions and other major US businesses," he said.
"The impact of this global cyber threat has been significant. Botnets have caused over $9bn in losses to US victims and over $110bn in losses globally. Approximately 500 million computers are infected globally each year, translating into 18 victims per second."
Demarest added this is doubly troubling as many of the botnets are currently rentable and could be used by a variety of criminals or terrorist organisations.
"As you well know, we face cyber threats from state-sponsored hackers, hackers for hire, organised cyber syndicates and terrorists. They seek our state secrets, our trade secrets, our technology and our ideas – things of incredible value to all of us," he said.
"They may seek to strike our critical infrastructure and our economy. The threat is so dire that cyber security has topped the Director of National Intelligence's list of global threats for the second consecutive year."
The FBI assistant director's claim follows the discovery of a new Energetic Bear hack campaign targeting critical infrastructure. The threat was so severe that at the start of July the US Industrial Control Systems Cyber Emergency Response Team (ICS-CERT) issued a warning urging firms involved in critical infrastructure to check their systems.
Demarest said the FBI is already developing new technologies and techniques to help mitigate the growing threat, but argued that increased collaboration between law enforcement agencies and the public and private sector is needed to deal with the problem.
"The FBI's overall goal is to remove, reduce, and prevent cyber crime by attacking the threat through the identification of the most significant cyber criminal actors. Our success can only be attained through co-ordination of our overall cyber criminal strategy amongst all FBI Cyber Division's existing and emerging entities," he said.
"The FBI cyber criminal strategy also includes working closely with our international partners to develop a holistic assessment of the threat posed by cyber criminals and organisations to partner countries.
"Through this collaborative process, the FBI hopes to launch aggressive and comprehensive mitigation strategies through joint investigations and operational partnerships with law enforcement partners, private industry, and academia."
Demarest highlighted the recent success of the recent international Gameover Zeus takedown as proof of his claim. "In June 2014, the FBI announced a multinational effort to disrupt the Gameover Zeus botnet, the most sophisticated botnet that the FBI and its allies had ever attempted to disrupt," he said.
"This effort to disrupt it involved impressive co-operation with the private sector and international law enforcement. The FBI is proud of these successes, but we recognise that we must constantly strive to be more efficient and effective. Just as our adversaries continue to evolve, so too must the FBI.
Experts within the security community have been less positive about the Gameover Zeus operation, though. Speaking to V3 after the takedown many warned the operation could spur the botnet's owners to develop more dangerous attack strategies.
The warnings proved right on 11 July when an evolved, more resilient version of the Gameover Zeus botnet was discovered.