The most important change to security in the .NET Framework 4.5 is in strong naming. See Enhanced Strong Naming for a description of those changes.
The .NET Framework provides a two-tier security model for managed applications. Windows 8.x Store apps run in a Windows security container that limits access to resources. Within that container, managed applications run fully trusted. From a code access security (CAS) perspective, there is nothing a developer can do to elevate privileges. For information about the privileges granted by Windows, see App capability declarations (Windows Store apps) in the Windows Dev Center. For information about creating a Windows 8.x Store app, see Create your first Windows Store app using C# or Visual Basic.