A new version of MyDoom discovered at the weekend appears months after the last iteration of the long-running series of worms. MyDoom-AI (alias MyDoom-AE) normally spreads by emails with some featuring sexually explicit images. It claims attachments contains passwords for adult websites.
In fact, they contain malicious code that disables security software and turns PCs into drones for zombie attack networks. Its spread so far is limited but a fresh variant of MyDoom three months after the last batch is bad news. November's infamous Bofra worm was initially classified as a variant of MyDoom, but even excluding this more than 30 variants of the worm have been created since its debut in January 2004.
By comparison Agobot (AKA Gaobot or Phatbot) backdoor - which like MyDoom is often used to commandeer vulnerable Windows PCs to distribute spam or mount DDoS attacks - has hundreds of variants (example here). The source code for Gaobot is in the public domain and it has been modified and reposted widely. Anti-virus firm McAfee reckons virus writers are creating 150 zombie programs a week. ®