The main reason for its existence is to send out millions of spam messages per day, and it occasionally also harvests information such as email addresses in order to include them in new spam campaigns.
As the Virut botnet a week ago, this most resilient of botnets has recently been rented by cyber crooks wishing to infect computers with malware - in this case, the backdoor Cridex worm.
"The attackers have managed to host a malicious HTML file at a legitimate web site, which has been compromised. This file would then redirect the user to a Blackhole exploit kit, which would deliver W32.Cridex to the compromised computer," Symantec researchers explained.
The victims would be taken to the compromised web site if they clicked on links contained in bogus spam emails such as this one:
The majority of computers enslaved in the Cutwail botnet are at this time located in the U.S., India, the Russian Federation and Mexico. The servers hosting the Blackhole exploit kit are in Germany, the Russian Federation and Lithuania.
As always, users are advised to keep their OS and software updated, as well as avoid clicking on links contained in unsolicited emails.