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.

1 files changed, 38 insertions, 37 deletions

diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index 1f765f302c..b42ffe50d8 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -17,48 +17,49 @@ module Rails
                     :session_options, :time_zone, :reload_classes_only_on_change,
                     :beginning_of_week, :filter_redirect, :x, :enable_dependency_loading,
                     :read_encrypted_secrets, :log_level, :content_security_policy_report_only,
-                    :require_master_key
+                    :content_security_policy_nonce_generator, :require_master_key
 
       attr_reader :encoding, :api_only, :loaded_config_version
 
       def initialize(*)
         super
-        self.encoding                        = Encoding::UTF_8
-        @allow_concurrency                   = nil
-        @consider_all_requests_local         = false
-        @filter_parameters                   = []
-        @filter_redirect                     = []
-        @helpers_paths                       = []
-        @public_file_server                  = ActiveSupport::OrderedOptions.new
-        @public_file_server.enabled          = true
-        @public_file_server.index_name       = "index"
-        @force_ssl                           = false
-        @ssl_options                         = {}
-        @session_store                       = nil
-        @time_zone                           = "UTC"
-        @beginning_of_week                   = :monday
-        @log_level                           = :debug
-        @generators                          = app_generators
-        @cache_store                         = [ :file_store, "#{root}/tmp/cache/" ]
-        @railties_order                      = [:all]
-        @relative_url_root                   = ENV["RAILS_RELATIVE_URL_ROOT"]
-        @reload_classes_only_on_change       = true
-        @file_watcher                        = ActiveSupport::FileUpdateChecker
-        @exceptions_app                      = nil
-        @autoflush_log                       = true
-        @log_formatter                       = ActiveSupport::Logger::SimpleFormatter.new
-        @eager_load                          = nil
-        @secret_token                        = nil
-        @secret_key_base                     = nil
-        @api_only                            = false
-        @debug_exception_response_format     = nil
-        @x                                   = Custom.new
-        @enable_dependency_loading           = false
-        @read_encrypted_secrets              = false
-        @content_security_policy             = nil
-        @content_security_policy_report_only = false
-        @require_master_key                  = false
-        @loaded_config_version               = nil
+        self.encoding                            = Encoding::UTF_8
+        @allow_concurrency                       = nil
+        @consider_all_requests_local             = false
+        @filter_parameters                       = []
+        @filter_redirect                         = []
+        @helpers_paths                           = []
+        @public_file_server                      = ActiveSupport::OrderedOptions.new
+        @public_file_server.enabled              = true
+        @public_file_server.index_name           = "index"
+        @force_ssl                               = false
+        @ssl_options                             = {}
+        @session_store                           = nil
+        @time_zone                               = "UTC"
+        @beginning_of_week                       = :monday
+        @log_level                               = :debug
+        @generators                              = app_generators
+        @cache_store                             = [ :file_store, "#{root}/tmp/cache/" ]
+        @railties_order                          = [:all]
+        @relative_url_root                       = ENV["RAILS_RELATIVE_URL_ROOT"]
+        @reload_classes_only_on_change           = true
+        @file_watcher                            = ActiveSupport::FileUpdateChecker
+        @exceptions_app                          = nil
+        @autoflush_log                           = true
+        @log_formatter                           = ActiveSupport::Logger::SimpleFormatter.new
+        @eager_load                              = nil
+        @secret_token                            = nil
+        @secret_key_base                         = nil
+        @api_only                                = false
+        @debug_exception_response_format         = nil
+        @x                                       = Custom.new
+        @enable_dependency_loading               = false
+        @read_encrypted_secrets                  = false
+        @content_security_policy                 = nil
+        @content_security_policy_report_only     = false
+        @content_security_policy_nonce_generator = nil
+        @require_master_key                      = false
+        @loaded_config_version                   = nil
       end
 
       def load_defaults(target_version)