Last week, a fairly severe flaw in Yahoo Messenger was made public, affecting all new versions of the client. The flaw was severe enough to result in system compromise, but now the issue has been resolved.
Yahoo has released a new version of the Messenger suite, specifically to address some security flaws which include the webcam exploit. A lesser denial of service flaw was also corrected. If you are a YIM user, you should download the 18.104.22.1686 client.