Strict-Transport-Security Header

Checks if your site, when running with HTTPS, contains the Strict-Transport-Security Header (HSTS).

How to fix this health check

This health check can be fixed by adding the Strict-Transport-Security header to responses. The header tells browsers that future requests should be made over HTTPS only.

Enabling HSTS on a domain will cause browsers to only use HTTPS (not HTTP) to communicate with your site. Only enable HSTS on domains that can, and should, use HTTPS exclusively.

Using the UseHsts extension method

ASP.NET Core implements HSTS with the UseHsts extension method.

You can add UseHsts after the env.IsDevelopment() check in Startup.cs.

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseHsts();
    }
    //...
}

This example only enables HSTS if the app is not running in development mode. UseHsts isn't recommended in development because the HSTS settings are highly cacheable by browsers.

Full details of UseHsts, and additional configuration, can be found in the ASP.NET Core documentation.