This summer has seen a huge surge in the number of user computers remotely taken over by botnets.
The Shadowserver Foundation, which tracks "zombie" botnet PC numbers worldwide, said there was a three-fold increase in the last three months.
By the end of August, it said there were more than 450,000 computers which were part of worldwide botnets controlled by cyber criminals. In June, the number was a more modest 100,000.
The botnets can be used by criminals to spread spam and distribute malware to other users across the web, without the owners of botnet PCs knowing their machine is being remotely controlled.
The jump in zombie numbers is said to be linked to widespread attacks that inject malicious code onto legitimate websites, which then infects the machines of visitors.
Brazilian authorities recently charged a man for his alleged part in "sub-letting" a botnet he controlled with others to other cyber criminals for £20,000 a time.
The Shadowserver Foundation is a group of volunteer security professionals who track and measure botnets and other threats to help law enforcement authorities.