The vulnerability is caused by an extra character in the Microsoft Active Template Library (ATL) It could lead to remote code execution attacks. The mandatory upgrade will start in the middle of September and will force users to apply the latest Live Messenter 14.0.8089 if they want to use Microsoft s instant messaging service. The vulnerability affects users of Windows Live Messenger 8.1 and 8.5.1. The upgrade is scheduled to start later this month and will be mandatory.
Source: https://threatpost.com/microsoft-ship-mandatory-live-messenger-security-patch-090309/72238/