From 31abee0341cb9d19f0234da7b42dddbabfcd1d4a Mon Sep 17 00:00:00 2001
From: Andrew White <andrew.white@unboxed.co>
Date: Fri, 16 Feb 2018 13:21:48 +0000
Subject: Add support for automatic nonce generation for Rails UJS

Because the UJS library creates a script tag to process responses it
normally requires the script-src attribute of the content security
policy to include 'unsafe-inline'.

To work around this we generate a per-request nonce value that is
embedded in a meta tag in a similar fashion to how CSRF protection
embeds its token in a meta tag. The UJS library can then read the
nonce value and set it on the dynamically generated script tag to
enable it to execute without needing 'unsafe-inline' enabled.

Nonce generation isn't 100% safe - if your script tag is including
user generated content in someway then it may be possible to exploit
an XSS vulnerability which can take advantage of the nonce. It is
however an improvement on a blanket permission for inline scripts.

It is also possible to use the nonce within your own script tags by
using `nonce: true` to set the nonce value on the tag, e.g

    <%= javascript_tag nonce: true do %>
      alert('Hello, World!');
    <% end %>

Fixes #31689.
---
 .../assets/javascripts/rails-ujs/utils/ajax.coffee |  4 +++-
 .../assets/javascripts/rails-ujs/utils/csp.coffee  |  4 ++++
 actionview/lib/action_view/helpers.rb              |  2 ++
 actionview/lib/action_view/helpers/csp_helper.rb   | 24 ++++++++++++++++++++
 .../lib/action_view/helpers/javascript_helper.rb   | 11 +++++++++
 actionview/test/ujs/public/test/call-ajax.js       |  3 +--
 actionview/test/ujs/server.rb                      | 26 ++++++++++++++++------
 .../test/ujs/views/layouts/application.html.erb    |  7 +++---
 8 files changed, 68 insertions(+), 13 deletions(-)
 create mode 100644 actionview/app/assets/javascripts/rails-ujs/utils/csp.coffee
 create mode 100644 actionview/lib/action_view/helpers/csp_helper.rb

(limited to 'actionview')

diff --git a/actionview/app/assets/javascripts/rails-ujs/utils/ajax.coffee b/actionview/app/assets/javascripts/rails-ujs/utils/ajax.coffee
index cc0e037428..2a8f5659e3 100644
--- a/actionview/app/assets/javascripts/rails-ujs/utils/ajax.coffee
+++ b/actionview/app/assets/javascripts/rails-ujs/utils/ajax.coffee
@@ -1,7 +1,8 @@
+#= require ./csp
 #= require ./csrf
 #= require ./event
 
-{ CSRFProtection, fire } = Rails
+{ cspNonce, CSRFProtection, fire } = Rails
 
 AcceptHeaders =
   '*': '*/*'
@@ -65,6 +66,7 @@ processResponse = (response, type) ->
       try response = JSON.parse(response)
     else if type.match(/\b(?:java|ecma)script\b/)
       script = document.createElement('script')
+      script.nonce = cspNonce()
       script.text = response
       document.head.appendChild(script).parentNode.removeChild(script)
     else if type.match(/\b(xml|html|svg)\b/)
diff --git a/actionview/app/assets/javascripts/rails-ujs/utils/csp.coffee b/actionview/app/assets/javascripts/rails-ujs/utils/csp.coffee
new file mode 100644
index 0000000000..8d2d6ce447
--- /dev/null
+++ b/actionview/app/assets/javascripts/rails-ujs/utils/csp.coffee
@@ -0,0 +1,4 @@
+# Content-Security-Policy nonce for inline scripts
+cspNonce = Rails.cspNonce = ->
+  meta = document.querySelector('meta[name=csp-nonce]')
+  meta and meta.content
diff --git a/actionview/lib/action_view/helpers.rb b/actionview/lib/action_view/helpers.rb
index 46f20c4277..8cc8013718 100644
--- a/actionview/lib/action_view/helpers.rb
+++ b/actionview/lib/action_view/helpers.rb
@@ -13,6 +13,7 @@ module ActionView #:nodoc:
     autoload :CacheHelper
     autoload :CaptureHelper
     autoload :ControllerHelper
+    autoload :CspHelper
     autoload :CsrfHelper
     autoload :DateHelper
     autoload :DebugHelper
@@ -46,6 +47,7 @@ module ActionView #:nodoc:
     include CacheHelper
     include CaptureHelper
     include ControllerHelper
+    include CspHelper
     include CsrfHelper
     include DateHelper
     include DebugHelper
diff --git a/actionview/lib/action_view/helpers/csp_helper.rb b/actionview/lib/action_view/helpers/csp_helper.rb
new file mode 100644
index 0000000000..e2e065c218
--- /dev/null
+++ b/actionview/lib/action_view/helpers/csp_helper.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module ActionView
+  # = Action View CSP Helper
+  module Helpers #:nodoc:
+    module CspHelper
+      # Returns a meta tag "csp-nonce" with the per-session nonce value
+      # for allowing inline <script> tags.
+      #
+      #   <head>
+      #     <%= csp_meta_tag %>
+      #   </head>
+      #
+      # This is used by the Rails UJS helper to create dynamically
+      # loaded inline <script> elements.
+      #
+      def csp_meta_tag
+        if content_security_policy?
+          tag("meta", name: "csp-nonce", content: content_security_policy_nonce)
+        end
+      end
+    end
+  end
+end
diff --git a/actionview/lib/action_view/helpers/javascript_helper.rb b/actionview/lib/action_view/helpers/javascript_helper.rb
index dd2cd57ac3..acc50f8a62 100644
--- a/actionview/lib/action_view/helpers/javascript_helper.rb
+++ b/actionview/lib/action_view/helpers/javascript_helper.rb
@@ -63,6 +63,13 @@ module ActionView
       #   <%= javascript_tag defer: 'defer' do -%>
       #     alert('All is good')
       #   <% end -%>
+      #
+      # If you have a content security policy enabled then you can add an automatic
+      # nonce value by passing +nonce: true+ as part of +html_options+. Example:
+      #
+      #   <%= javascript_tag nonce: true do -%>
+      #     alert('All is good')
+      #   <% end -%>
       def javascript_tag(content_or_options_with_block = nil, html_options = {}, &block)
         content =
           if block_given?
@@ -72,6 +79,10 @@ module ActionView
             content_or_options_with_block
           end
 
+        if html_options[:nonce] == true
+          html_options[:nonce] = content_security_policy_nonce
+        end
+
         content_tag("script".freeze, javascript_cdata_section(content), html_options)
       end
 
diff --git a/actionview/test/ujs/public/test/call-ajax.js b/actionview/test/ujs/public/test/call-ajax.js
index 49e64cad5c..4d0bfb0806 100644
--- a/actionview/test/ujs/public/test/call-ajax.js
+++ b/actionview/test/ujs/public/test/call-ajax.js
@@ -8,7 +8,6 @@ module('call-ajax', {
 })
 
 asyncTest('call ajax without "ajax:beforeSend"', 1, function() {
-
   var link = $('#qunit-fixture a')
   link.bindNative('click', function() {
     Rails.ajax({
@@ -21,7 +20,7 @@ asyncTest('call ajax without "ajax:beforeSend"', 1, function() {
   })
 
   link.triggerNative('click')
-  setTimeout(function() { start() }, 13)
+  setTimeout(function() { start() }, 50)
 })
 
 })()
diff --git a/actionview/test/ujs/server.rb b/actionview/test/ujs/server.rb
index 7d1bab4b2a..48e9bcb65f 100644
--- a/actionview/test/ujs/server.rb
+++ b/actionview/test/ujs/server.rb
@@ -23,18 +23,30 @@ module UJS
     config.public_file_server.enabled = true
     config.logger = Logger.new(STDOUT)
     config.log_level = :error
+
+    config.content_security_policy do |policy|
+      policy.default_src :self, :https
+      policy.font_src    :self, :https, :data
+      policy.img_src     :self, :https, :data
+      policy.object_src  :none
+      policy.script_src  :self, :https
+      policy.style_src   :self, :https
+    end
+
+    config.content_security_policy_nonce_generator = ->(req) { SecureRandom.base64(16) }
   end
 end
 
 module TestsHelper
   def test_to(*names)
-    names = ["/vendor/qunit.js", "settings"] + names
-    names.map { |name| script_tag name }.join("\n").html_safe
-  end
+    names = names.map { |name| "/test/#{name}.js" }
+    names = %w[/vendor/qunit.js /test/settings.js] + names
 
-  def script_tag(src)
-    src = "/test/#{src}.js" unless src.index("/")
-    %(<script src="#{src}" type="text/javascript"></script>).html_safe
+    capture do
+      names.each do |name|
+        concat(javascript_include_tag(name))
+      end
+    end
   end
 end
 
@@ -56,7 +68,7 @@ class TestsController < ActionController::Base
     elsif params[:iframe]
       payload = JSON.generate(data).gsub("<", "&lt;").gsub(">", "&gt;")
       html = <<-HTML
-        <script>
+        <script nonce="#{request.content_security_policy_nonce}">
           if (window.top && window.top !== window)
             window.top.jQuery.event.trigger('iframe:loaded', #{payload})
         </script>
diff --git a/actionview/test/ujs/views/layouts/application.html.erb b/actionview/test/ujs/views/layouts/application.html.erb
index c787e77b84..8f6f6fc17f 100644
--- a/actionview/test/ujs/views/layouts/application.html.erb
+++ b/actionview/test/ujs/views/layouts/application.html.erb
@@ -2,9 +2,10 @@
 <html id="html">
   <head>
     <title><%= @title %></title>
+    <%= csp_meta_tag %>
     <link href="/vendor/qunit.css" media="screen" rel="stylesheet" type="text/css" media="screen, projection" />
     <script src="/vendor/jquery-2.2.0.js" type="text/javascript"></script>
-    <script>
+    <%= javascript_tag nonce: true do %>
       // This is for test in override.js.
       // Must go before rails-ujs.
       document.addEventListener('rails:attachBindings', function() {
@@ -15,8 +16,8 @@
           e.preventDefault();
         });
       });
-    </script>
-    <%= script_tag "/rails-ujs.js" %>
+    <% end %>
+    <%= javascript_include_tag "/rails-ujs.js" %>
   </head>
 
   <body id="body">
-- 
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