If you use Yahoo Messenger, then you need to update it to prevent the hackers and crackers from getting you. F-Secure says that there are already easy to obtain ways to use the 2 vulnerabilities to let hackers, script kiddies or whatever you want to call them take control of your computer:
Very accurate and script-kiddie-friendly exploits are publicly available for both vulnerablities. It is possible that crimeware distributors will start exploiting this for drive-by downloads.
Go to Yahoo's security update page for this problem if you want to see details about it. You can also go straight to the Yahoo Messenger download page to get the latest version to fix this problem.