Fix XSS vulnerability in `translate` helper

Prior to this commit, when a translation key indicated that the
translation text was HTML, the value returned by `I18n.translate` would
always be marked as `html_safe`.  However, the value returned by
`I18n.translate` could be an untrusted value directly from
`options[:default]`.

This commit ensures values directly from `options[:default]` are not
marked as `html_safe`.

actionview/lib/action_view/helpers/translation_helper.rb

@@ -76,13 +76,20 @@ def translate(key, **options)
 
         if html_safe_translation_key?(key)
           html_safe_options = options.dup
+
           options.except(*I18n::RESERVED_KEYS).each do |name, value|
             unless name == :count && value.is_a?(Numeric)
               html_safe_options[name] = ERB::Util.html_escape(value.to_s)
             end
           end
+
+          html_safe_options[:default] = MISSING_TRANSLATION unless html_safe_options[:default].blank?
+
           translation = I18n.translate(scope_key_by_partial(key), **html_safe_options.merge(raise: i18n_raise))
-          if translation.respond_to?(:map)
+
+          if translation.equal?(MISSING_TRANSLATION)
+            options[:default].first
+          elsif translation.respond_to?(:map)
             translation.map { |element| element.respond_to?(:html_safe) ? element.html_safe : element }
           else
             translation.respond_to?(:html_safe) ? translation.html_safe : translation
@@ -121,6 +128,9 @@ def localize(object, **options)
       alias :l :localize
 
       private
+        MISSING_TRANSLATION = Object.new
+        private_constant :MISSING_TRANSLATION
+
         def scope_key_by_partial(key)
           stringified_key = key.to_s
           if stringified_key.first == "."

actionview/test/template/translation_helper_test.rb

@@ -217,6 +217,13 @@ def test_translate_with_last_default_not_named_html
     assert_equal false, translation.html_safe?
   end
 
+  def test_translate_does_not_mark_unsourced_string_default_as_html_safe
+    untrusted_string = "<script>alert()</script>"
+    translation = translate(:"translations.missing", default: [:"translations.missing_html", untrusted_string])
+    assert_equal untrusted_string, translation
+    assert_not_predicate translation, :html_safe?
+  end
+
   def test_translate_with_string_default
     translation = translate(:'translations.missing', default: "A Generic String")
     assert_equal "A Generic String", translation