The JS-Yamanner worm spreads when a Windows user accesses Yahoo! Mail to open an email sent by the worm. The attack works because of a vulnerability in Yahoo! Mail that enables scripts embedded within HTML emails to be run within a user’s browser instead of being blocked.

Once executed, the worm forwards itself to an infected users' contacts on Yahoo! Mail. It also harvests these address and sends them to a remote internet server. Only contacts with an email address of either @yahoo.com or @yahoogroups.com are hit by this behaviour.

Infected emails commonly have the subject line "New Graphic Site" and are spoofed so as to appear from "av3@yahoo.com". Users who open infected emails will be redirected to a webpage at www.av3.net/index.htm.