Microsoft pushed out an emergency update for a critical security vulnerability that could allow an attacker to gain complete control of your system. The flaw affects all systems from Windows Vista and up including the Windows 10 Technical Preview.
The exploit works if you open a document or visit a webpage that contains malicious embedded OpenType fonts. The fonts then take advantage of a weakness in how the Windows Adobe Type Manager Library handles OpenType fonts. OpenType is a font file format developed by Adobe and Microsoft.
The security update is available immediately through the Windows Update mechanism. If you’re set up for automatic updates, your PC should’ve downloaded the update, if it hasn’t already. Anyone doing manual updates should check Windows Update immediately.
To verify if your PC has the critical update on Windows 8.1 (Windows Vista and 7 users will have a similar process), open the Control Panel, search for “Windows Update”, and then choose View update history. Towards the top of the list you should see a security update with the number KB3079904.