In fact, its rather convoluted action was covered under the security advisory MS04-013, released in April.
Wallons infection process is complicated. Unlike the ordinary e-mail worm that arrives in an attachment to a message, Wallon appears as a link in a message to a Yahoo page. But with redirection, the Yahoo connection leads to another page that delivers an encrypted link to yet another page that delivers a special downloader application.
Microsoft provided a security patch for this vulnerability in April and suggested its application for all currently supported Windows versions. The company describes the update as "critical" and recommends it for all Windows variants, starting with Windows 98, even for systems where Outlook Express is not the default e-mail reader.
The downloader app is activated by a call to the Windows Media Player, so when the user enters a media-rich site or views some streaming content, the actual worm is finally downloaded. It then proceeds to perform a series of actions to propagate itself, the services report.
Microsofts latest patch release addresses a similarly convoluted social engineering mechanism with advisory MS04-015, titled "Vulnerability in Help and Support Center Could Allow Remote Code Execution." In this case, users are directed to a malicious Web page where they click on a link and follow directions. The actual attack occurs only after they perform the actions.
Editors Note: This story was updated to correct the related bulletin information.