Anonymous supporters queuing up to participate in denial-of-service attacks are being tricked into installing ZeuS botnet clients.
Hacktivists grabbed what they thought was the Slowloris tool, which is designed to flood websites with open connections and ultimately knock them offline. However, the download included a strain of ZeuS, which promptly installed itself on their Microsoft Windows machines.
The Trojan will carry out the distributed attacks, but that's not all it does - it'll also steal users' online banking credentials, webmail logins, and cookies.
The deception began on 20 January, the same day as the FBI Megaupload raid, Symantec reports.
Malware pedlars swiped the template of an Anonymous guide to launching denial-of-service attacks from Pastebin, modified it to include a link to the nobbled build of Slowloris, and reposted the message on Pastebin to snare victims.
Anonymous is normally highly antagonistic to white-hat security firms, especially Symantec. This time however Twitter accounts maintained by the hacktivist collective were happy to endorse Symantec's warning.
Twit AnonymousIRC wrote: "Anonymous supporters tricked into installing Zeus trojan. This MUSTN'T happen. Be careful what you post and click on!" ®