"Microsoft wants customers to be aware that exploit code is now public and urges them to patch their systems," a company spokeswoman said in an e-mail.Read more at InformationWeek
To prevent a widespread attack, Microsoft went proactive and made updates mandatory for MSN Messenger users.
Microsoft prevents exploiting of flaw in MSN Messenger

Microsoft took a drastic action to prevent malicious users from exploiting a flaw in its MSN Messenger software - basically users will not be able to use the MSN network until they have upgraded their MSN client to the latest version.