Add rename_csp_helper_nonce_attribute actionview configuration

Adds a configuration to rename the csp helper attribute name.

It's disabled by default currently until the JS libraries are updated to the
new attribute name and Rails can ship with a new default attribute name.

Fixes rails#51580

actionview/CHANGELOG.md

@@ -1,3 +1,17 @@
+*   Add a disabled configuration `rename_csp_helper_nonce_attribute` to rename the csp_meta_tag helper nonce attribute name
+    If enabled, it renames the `content` attribute to `nonce` to avoid certain kinds of value exfiltration attacks.
+
+    ```
+    app.config.action_view.rename_csp_helper_nonce_attribute = true
+    <%= csp_meta_tag %>
+    # renders
+    <meta name="csp-nonce" nonce="..." />
+    # instead of
+    <meta name="csp-nonce" content="..." />
+    ```
+
+    *Niklas Häusele*
+
 *   Add queries count to template rendering instrumentation
 
     ```

actionview/lib/action_view/helpers/csp_helper.rb

@@ -4,6 +4,9 @@ module ActionView
   module Helpers # :nodoc:
     # = Action View CSP \Helpers
     module CspHelper
+      mattr_accessor :rename_csp_helper_nonce_attribute
+      self.rename_csp_helper_nonce_attribute = nil
+
       # Returns a meta tag "csp-nonce" with the per-session nonce value
       # for allowing inline <script> tags.
       #
@@ -17,7 +20,8 @@ module CspHelper
       def csp_meta_tag(**options)
         if content_security_policy?
           options[:name] = "csp-nonce"
-          options[:content] = content_security_policy_nonce
+          nonce_attribute_name = rename_csp_helper_nonce_attribute ? :nonce : :content
+          options[nonce_attribute_name] = content_security_policy_nonce
           tag("meta", options)
         end
       end

actionview/lib/action_view/railtie.rb

@@ -14,9 +14,15 @@ class Railtie < Rails::Engine # :nodoc:
     config.action_view.image_decoding = nil
     config.action_view.apply_stylesheet_media_default = true
     config.action_view.prepend_content_exfiltration_prevention = false
+    config.action_view.rename_csp_helper_nonce_attribute = false
 
     config.eager_load_namespaces << ActionView
 
+    config.after_initialize do |app|
+      ActionView::Helpers::CspHelper.rename_csp_helper_nonce_attribute =
+        app.config.action_view.delete(:rename_csp_helper_nonce_attribute)
+    end
+
     config.after_initialize do |app|
       ActionView::Helpers::FormTagHelper.embed_authenticity_token_in_remote_forms =
         app.config.action_view.delete(:embed_authenticity_token_in_remote_forms)

actionview/test/template/csp_helper_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "abstract_unit"
+require "active_support/core_ext/object/with"
 
 class CspHelperWithCspEnabledTest < ActionView::TestCase
   tests ActionView::Helpers::CspHelper
@@ -13,7 +14,19 @@ def content_security_policy?
     true
   end
 
-  def test_csp_meta_tag
+  def test_csp_meta_tag_uses_nonce_attribute_name_with_helper_nonce_attribute_enabled
+    ActionView::Helpers::CspHelper.with(rename_csp_helper_nonce_attribute: true) do
+      assert_equal "<meta name=\"csp-nonce\" nonce=\"iyhD0Yc0W+c=\" />", csp_meta_tag
+    end
+  end
+
+  def test_csp_meta_tag_uses_content_attribute_name_with_helper_nonce_attribute_disabled
+    ActionView::Helpers::CspHelper.with(rename_csp_helper_nonce_attribute: false) do
+      assert_equal "<meta name=\"csp-nonce\" content=\"iyhD0Yc0W+c=\" />", csp_meta_tag
+    end
+  end
+
+  def test_csp_meta_tag_with_helper_nonce_attribute_default_setting
     assert_equal "<meta name=\"csp-nonce\" content=\"iyhD0Yc0W+c=\" />", csp_meta_tag
   end