Can't access GeoServer after CORS filter

To enable CORS filter I added the following in web.xml file in webapps/Geoserver/WEB-INF

 cross-origin
 org.eclipse.jetty.servlets.CrossOriginFilter

...

    cross-origin
    /*

After adding above, I restarted GeoServer. Now, when I access GeoServer, I get following error:

HTTP ERROR: 503

Problem accessing /geoserver/. Reason:

Service Unavailable

Answer

We had the same issue, after enabling CORS by removing the comments in web.xml original file supplied with GeoServer, GeoServer was no longer starting properly in any configuration (we tried using Mac OS X installer, with the platform independent binary and using Apache/Tomcat).

Finally, we managed to get the configuration to work using Apache/Tomcat but not using the default filter org.eclipse.jetty.servlets.CrossOriginFilter specified in web.xml supplied file but the Tomcat org.apache.catalina.filters.CorsFilter filter, as per https://enable-cors.org/server_tomcat.html (see also jgrocha answer at https://stackoverflow.com/questions/22363192/cors-tomcat-geoserver):

  CorsFilter
  org.apache.catalina.filters.CorsFilter


  CorsFilter
  /*