The Windows Live Messenger team announced a phased upgrade to the quietly released 14.0.8089.726 build, which addresses vulnerabilities in Microsoft's Active Template Library (ATL) as cited in Microsoft Security Advisory 973882. The upgrade also comes with unspecified code fixes and feature enhancements.
Users of Windows Live Messenger 8.1, 8.5 and older 14.0 versions will be asked and later forced to install the update.
First Phase, Optional Upgrade:
1) Starting Aug. 25, customers using versions 8.1 or 8.5 were asked to upgrade their client.
2) Starting early Oct., all customers using versions 14.0 (but not the latest release 14.0.8089) will be asked to upgrade their client.
The upgrade at this time is optional. Customers who haven't upgraded during the optional phase will be required to do so during the second phase.
Second Phase, Mandatory Upgrade:
1) Starting mid-Sept., all customers using Messenger 8.1 or 8.5 will be required to upgrade their version of Windows Live Messenger.
2) Starting late Oct., all customers using Messenger 14.0 will be required to upgrade their version of Windows Live Messenger.
Of course, you don't have to wait for the upgrade notification to pop up and just grab the latest build from messenger.live.com.