More than a million PCs under the control of spammers are threatening the US national security, its economy and its information infrastructure, according to the FBI.
The discovery was made by Operation Bot Roast, which is an initiative aimed at revealing the scale of the botnet problem and prosecuting those responsible. It is being carried out in conjunction with Carnegie Mellon University, Microsoft and the International Botnet Task Force.
Botnets are collections of computers -- known as bots or zombies -- whose respective owners have no idea their PCs have been hijacked and are being used for identity theft, denial-of-service attacks and the transmission of malware and spam.
The FBI claims that botnets are a "growing threat to national security, the national information infrastructure, and the economy" in the US. The assistant director for the FBI's Cyber Division, James Finch, urged computer users last week to "protect themselves from botnets and the associated schemes by practising strong computer security habits".
The organisations participating in Operation Bot Roast are in the process of trying to contact victims to inform them of their computers' compromised status.
As part of Operation Bot Roast, American authorities in May arrested Robert Alan Soloway, the alleged "Seattle Spammer". Soloway stands accused of using a large botnet to circulate tens of millions of spam e-mails. Despite federal laws against such behaviour, the US remains the most prolific originator of spam in the world.
David Meyer reported for ZDNet UK from London