http://www.theregister.co.uk/2006/06/12/javscript_worm_targets_ya
hoo/
http://www.pcworld.com/news/article/0,aid,126048,00.asp
In a nutshell - it’s a flaw in Yahoo mail itself and so far there doesn’t
seem to be any cure. Other than replicating itself it doesn’t seem to
cause any damage, but if yahoo doesn’t fix it - the next worm could
be very bad.
I can’t find any info about it here on Avast, though. :-[