title: Using expression-filter in undertow is not working
body: <p>I'm using Wildfly 10 and I'm trying to add an expression filter in the undertow configuration to validate the secret value from mod_jk. It is however always returning error code 403. </p>
  
  <p>Below is my configuration in standalone-full.xml</p>
  
  <pre><code>&lt;subsystem xmlns="urn:jboss:domain:undertow:7.0" ...&gt;
  ...
  &lt;server name="default-server"&gt;
      &lt;http-listener name="default" socket-binding="http" redirect-socket="https" enable-http2="true"/&gt;
      &lt;ajp-listener name="ajp" socket-binding="ajp"/&gt;
      &lt;https-listener name="https" socket-binding="https" security-realm="ApplicationRealm" enable-http2="true"/&gt;
      &lt;host name="default-host" alias="localhost"&gt;
          ...
          &lt;!-- add the following with your AJP port (8009) --&gt;
          &lt;filter-ref name="secret-checker" predicate="equals(%p, 8009)"/&gt;
      &lt;/host&gt;
  &lt;/server&gt;
  ...
  &lt;filters&gt;
      &lt;!-- add the following with your credential (YOUR_AJP_SECRET) --&gt;
      &lt;expression-filter name="secret-checker" expression="not equals(%{r,secret}, 'verysecure') -&gt; response-code(403)"/&gt;
  &lt;/filters&gt;
  </code></pre>
  
  <p></p>
  
  <p>Below is the configuration in workers.properties for mod_jk.</p>
  
  <pre><code>worker.list=ajp13
  worker.ajp13.port=8009
  worker.ajp13.host=127.0.0.1
  worker.ajp13.type=ajp13
  worker.ajp13.secret=verysecure
  </code></pre>
  
  <p>I'm trying to mitigate the ghostcat vulnerability detailed on this link.
  <a href="https://access.redhat.com/solutions/4851251" rel="nofollow noreferrer">https://access.redhat.com/solutions/4851251</a></p>
  
  <p>Any help is appreciated. Thanks in advance.</p>