The attacking program could obtain your entire address-book, supplementing spammers¡¯ lucrative database of spam victims. And thanks to Yahoos recent integration of instant messaging within its web-mail site, it could also send instant messages to your friends impersonating you. ("You wanna do what to me?!") Many of your online accounts would then become vulnerable as well, since the attacker would have access to your password-resetting emails; this potentially means access to your financial accounts. All other Yahoo services that you use are also at risk, including Yahoo Photos or Flickr.
