It wouldn't be Halloween without the zombie-creating Storm malware up to some mischief.

The latest Storm-backed spam campaign invites e-mail recipients to visit a Halloween-themed Web site where they can download a dancing skeleton. What gets downloaded instead is a version of the Storm malware that turns unsuspecting users' PCs into members of the world's largest botnet. Members of these botnets are also known as zombies.

According to security vendor Marshal, the e-mail's embedded link is not to a URL but to an IP address. Users who click on the link to the Halloween Web site and don't have their browsers up to date with security patches could automatically become infected, Marshal says. Those who have current patches but click on the link to download the dancing skeleton could also become infected.

Storm's creators are nothing if not prolific. These malware writers jump on current event topics and seasonal happenings to lure e-mail users into visiting infected Web sites in attempts to grow their botnet, which some say now totals over 1 million members. Recent Storm spam campaigns have used Labor Day, the opening of the National Football League's season, and promises of YouTube video clips and current music snippets to trick recipients into clicking on Web links and becoming infected.

The malware's creators are also quick to deflect attempts by researchers to learn more about Storm. The worm can figure out which users are trying to probe its command-and-control servers and retaliates by launching distributed denial-of-service attacks against them, shutting down their Internet access for days, according to an IBM/ISS security professional.