A single botnet has grown so dramatically in the last two months that it now accounts for half of all the spam, according to new research from Marshal's TRACE team. The Srizbi botnet has steadily increased its network since the beginning of 2008 and is now the world's largest spam botnet.

"Srizbi is the single greatest spam threat we have ever seen," said Bradley Anstis, vice president of Products at Marshal. "At its peak, the highly publicized Storm botnet only accounted for 20 percent of spam. Srizbi now produces more spam than all the other botnets combined.

"Srizbi is estimated to comprise at least 300,000 compromised computers and sends more than 60 billion spam messages per day," Anstis continued. "Recently, the botnet has been used to promote a range of products including watches, pens and male enlargement pills. Srizbi also actively distributes copies of its own malware in malicious spam campaigns using social engineering ploys to trick recipients into infecting their computers.

"Microsoft recently announced its success combating the Storm botnet with their Malicious Software Removal Tool (MSRT)," said Anstis. "The challenge now is for the security industry to turn its sights on Srizbi and the other major botnets. We look forward to seeing Microsoft target Srizbi with MSRT in the near future."

In January, Marshal announced that the infamous Storm botnet had lost its place as the number one source of spam to the Mega-D botnet; otherwise known as Ozdok. The latest data shows that Srizbi's rise to prominence in February coincided with the decline of the Mega-D botnet after its control servers were taken offline for 10 days.

"It is difficult to say for certain why Mega-D went offline after we announced our findings," said Anstis. "It is probable the spammers got spooked and decided to lay low for a while. Security researchers were close to discovering their control servers when the plug was pulled. Typically, the spammers like the 'low and slow' approach, building their botnet up over time and trying to stay under the radar to avoid detection. It is an intriguing chain of events that as Mega-D went offline; Srizbi stepped in to fill the gap and hasn't looked back since."

Mega-D quickly bounced back and regularly jostles with the Rustock botnet for second place in spam statistics. Marshal's TRACE researchers have observed individual computers infected with the Rustock malware sending as many as 25,000 spam messages an hour.

"While other botnets such as Rustock might be smaller in terms of the number of compromised computers under their control they can be very efficient at sending spam," added Anstis.