Yahoo is shipping a security update that fixes a couple of holes in the way its Yahoo Messenger client handles webcams. At least one of the holes can result in a "buffer overflow" condition, a type of vulnerability that can often result in arbitrary code execution – meaning your PC could be completely compromised.
Here's what Yahoo's alert says:
"Some impacts of a buffer overflow might include the introduction of executable code, being involuntarily logged out of a Chat and/or Instant Messaging session, and the crash of an application such as Yahoo! Messenger. For this specific security issue, these impacts could only be possible if an attacker is successful in prompting the Messenger user to accept a webcam invitation."
Read more here at PC World