More information about this security flaw is available in the following bug: https://bugzilla.redhat.com/show_bug.cgi?id=2351766 Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.
Caddy bundles golang.org/x/net, but because of the way go bundling works it doesn't include the affected code in golang.org/x/net/http/httpproxy or golang.org/x/net/proxy.