Adding X-Frame-Options header to all pages in MVC 4 application

There’s no need for a custom HttpModule or ActionFilter if you need it for every page. https://developer.mozilla.org/en-US/docs/HTTP/X-Frame-Options details a much simpler solution:

To configure IIS to send the X-Frame-Options header, add this your site’s Web.config file:

<system.webServer>
  <!-- ... -->

  <httpProtocol>
    <customHeaders>
      <add name="X-Frame-Options" value="SAMEORIGIN" />
    </customHeaders>
  </httpProtocol>

  <!-- ... -->
</system.webServer>