A very recent (as of Monday, June 12) email harvesting worm has managed to successfully infect many users of Yahoo! Mail. The worm works simply by reading the email--you do not have to actually open the attached file. Fortunately the payload is relatively harmless; it simply harvests your email address and sends it to a server, then it replicates to all other @yahoo.com addresses in your contacts list. Note that the latest version of AT&T Yahoo! Mail Beta seems to be protected against the exploit used by the worm, but many people do not yet use the new Beta version of Yahoo! Mail.

If someone else has been successfully infected and the worm was replicated to your Yahoo! Mail inbox, you'll have an email from that person with the subject "New Graphic Site". The best way to prevent becoming infected yourself is to delete that email without reading it.

Since this is a very new worm, your anti-viral software might not yet have a fix for it. Symantec (Norton), for example, has an "intelligent update" that you can manually install, but their LiveUpdate won't install the fix until June 14. Symantec's Security response article has a fairly comprehensive description of the worm if you need more info: http://securityresponse.symantec.com/avcenter/venc/data/js.yamanner@m.ht...

Also, here is a good ZDNet article that describes the worm and its impact: http://www.zdnetasia.com/news/security/0,39044215,39367249,00.htm