The fall in global spam levels after the shutting of web hosting company McColo looks to be short-lived after researchers reported that the Srizbi botnet has been reactivated.

The Srizbi botnet is thought to consist of over 500,000 computers and is estimated to be responsible for up to 40% of the world's spam. Researchers at security consultants FireEye are now warning that it, and other botnets, have been reactivated.
 
"Srizbi has returned from the dead and has begun updating all its Bots with a fresh, new binary," it reported. "The worldwide update began just a few hours ago. In the coming days, many journalists and researchers will ask themselves: ‘How is it possible that the largest Botnet in the world was allowed to update itself, when a security firm had near complete control over it?' This is an interesting angle that we'll be exploring once all the technical facts are out on the table."

The botnet command and control systems were initially put back online in Estonia but have since been taken down.

The researchers also reported that the Rustock rootkit is also back in circulation and is sending a variety of spam based on social engineering and the sale of medical products.