ASP.NET no longer allows developers to specify `<pages enableViewStateMac="false"/>` or `<@Page EnableViewStateMac="false" %>`. The view state message authentication code (MAC) is now enforced for all requests with embedded view state. Only apps that explicitly set the EnableViewStateMac property to `false` are affected.
EnableViewStateMac must be assumed to be true, and any resulting MAC errors must be resolved (as explained in [this guidance](https://support.microsoft.com/en-us/kb/2915218), which contains multiple resolutions depending on the specifics of what is causing MAC errors).
