Hi,

If I understand correctly you actually don't want to cache specific responses (not bypass). The proxy_cache_bypass is only for if the response has already been cached and defines the behavior in which NGINX should serve the cached version to a client.

Therefore if I understand correctly, you should be using the upstream module for your origin definition and the proper variable will be available as $upstream_http_x_secured_page
- http://nginx.org/en/docs/http/ngx_http_upstream_module.html#var_upstream_http_

Then use the proxy_no_cache directive to determine what should and should not be cached in your configuration. If you want to simply check if the header exists and then not cache the response, you can add the upstream_http_ variable as a parameter.
- http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_no_cache

If you need to inspect the header, then look into using map instead to define the conditions you need in other to set or not set the proxy_no_cache to a value or not.

Please let me know if my understanding is correct.