/usr/lib/ruby/2.3.0/webrick/ is in libruby2.3 2.3.0-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/2.3.0/webrick/accesslog.rb
/usr/lib/ruby/2.3.0/webrick/cgi.rb
/usr/lib/ruby/2.3.0/webrick/compat.rb
/usr/lib/ruby/2.3.0/webrick/config.rb
/usr/lib/ruby/2.3.0/webrick/cookie.rb
/usr/lib/ruby/2.3.0/webrick/htmlutils.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/
/usr/lib/ruby/2.3.0/webrick/httpauth.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/authenticator.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/basicauth.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/digestauth.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/htdigest.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/htgroup.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/htpasswd.rb
/usr/lib/ruby/2.3.0/webrick/httpauth/userdb.rb
/usr/lib/ruby/2.3.0/webrick/httpproxy.rb
/usr/lib/ruby/2.3.0/webrick/httprequest.rb
/usr/lib/ruby/2.3.0/webrick/httpresponse.rb
/usr/lib/ruby/2.3.0/webrick/https.rb
/usr/lib/ruby/2.3.0/webrick/httpserver.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/
/usr/lib/ruby/2.3.0/webrick/httpservlet.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/abstract.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/cgi_runner.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/cgihandler.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/erbhandler.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/filehandler.rb
/usr/lib/ruby/2.3.0/webrick/httpservlet/prochandler.rb
/usr/lib/ruby/2.3.0/webrick/httpstatus.rb
/usr/lib/ruby/2.3.0/webrick/httputils.rb
/usr/lib/ruby/2.3.0/webrick/httpversion.rb
/usr/lib/ruby/2.3.0/webrick/log.rb
/usr/lib/ruby/2.3.0/webrick/server.rb
/usr/lib/ruby/2.3.0/webrick/ssl.rb
/usr/lib/ruby/2.3.0/webrick/utils.rb
/usr/lib/ruby/2.3.0/webrick/version.rb