title: <access sslFlags="SslRequireCert" /> using IIS Express -> HTTP Error 500.19
body: <p>I'm trying to test client certificate authentication with <code>IIS Express</code>. Is it possible or do I need to use <code>IIS</code>?</p>
  
  <p>I have modified my web.config like this:</p>
  
  <pre><code>&lt;configuration&gt;
      &lt;system.webServer&gt;
          &lt;security&gt;
              &lt;access sslFlags="SslRequireCert" /&gt;
          &lt;/security&gt;
      &lt;/system.webServer&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>Guide that I use:</p>
  
  <p><a href="https://dotnetcodr.com/2016/01/11/using-client-certificates-in-net-part-1-introduction/" rel="nofollow noreferrer">https://dotnetcodr.com/2016/01/11/using-client-certificates-in-net-part-1-introduction/</a></p>
  
  <p><code>.vs\config\applicationhost.config</code> looks like this, no deny or allowOverride set to false.:</p>
  
  <pre><code>&lt;security&gt;
      &lt;access sslFlags="None" /&gt;
      ...
  &lt;/security&gt;
  </code></pre>
  
  <p>Error:</p>
  
  <blockquote>
    <p>HTTP Error 500.19 - Internal Server Error The requested page cannot be
    accessed because the related configuration data for the page is
    invalid.</p>
    
    <p>This configuration section cannot be used at this path. This happens
    when the section is locked at a parent level. Locking is either by
    default (overrideModeDefault="Deny"), or set explicitly by a location
    tag with overrideMode="Deny" or the legacy allowOverride="false".</p>
  </blockquote>