The worm -- JS.Yamanner@m -- spreads from person to person when the user opens the e-mail which is originally sent by the worm. The worm then sends itself to the user's contacts that also use Yahoo Mail, while simultaneously sending those e-mail addresses to a remote server on the Internet.

Only those using contacts with an e-mail address that is @yahoo.com or @yahoogroups.com will be impacted by this. Symantec Security Response is currently categorizing JS.Yamanner as a Level 2 threat.

Kevin Hogan, senior manager Symantec Security Response, comments: 'This worm is a twist on the traditional mass mailing worms that we have seen in recent years, and is very much in line with the trend for threats that target personal information. Unlike its predecessors, which would require the user to open an attachment in order to launch and propagate, JS.Yamanner makes use of a security hole in the Yahoo Web mail program in order to spread to other Yahoo users. Users of Yahoo Mail Beta do not appear to be vulnerable to JS.Yamanner.'