Upgrade to Rails 6.0.0.

config/initializers/content_security_policy.rb

@@ -11,6 +11,8 @@
 #   policy.object_src  :none
 #   policy.script_src  :self, :https
 #   policy.style_src   :self, :https
+#   # If you are using webpack-dev-server then specify webpack-dev-server host
+#   policy.connect_src :self, :https, "http://localhost:3035", "ws://localhost:3035" if Rails.env.development?
 
 #   # Specify URI for violation reports
 #   # policy.report_uri "/csp-violation-report-endpoint"
@@ -19,6 +21,9 @@
 # If you are using UJS then enable automatic nonce generation
 # Rails.application.config.content_security_policy_nonce_generator = -> request { SecureRandom.base64(16) }
 
+# Set the nonce only to specific directives
+# Rails.application.config.content_security_policy_nonce_directives = %w(script-src)
+
 # Report CSP violations to a specified URI
 # For further information see the following documentation:
 # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only