This is a two step procedure. At first, you need to create a JKS file from the Let's Encrypt certificates. You may find a nice article on this here [1]. Once this is done, just use the JKS file for the HTTPS in Undertow. You may find how to do it in Wildfly manual [2]. [1] https://community.letsencrypt.org/t/tutorial-java-keystores-jks-with-lets... [2] https://docs.jboss.org/author/display/WFLY/WildFly+Elytron+Security#WildF... On Mon, Sep 3, 2018 at 4:12 PM Christophe Lehingue <clehingue(a)gmail.com&gt; wrote: