prevent combination of select and style tags with the HTML4 parser

lib/rails/html/sanitizer.rb

@@ -141,8 +141,25 @@ def sanitize_css(style_string)
 
       private
 
+      def loofah_using_html5?
+        # future-proofing, see https://github.com/flavorjones/loofah/pull/239
+        Loofah.respond_to?(:html5_mode?) && Loofah.html5_mode?
+      end
+
+      def remove_safelist_tag_combinations(tags)
+        if !loofah_using_html5? && tags.include?("select") && tags.include?("style")
+          warn("WARNING: #{self.class}: removing 'style' from safelist, should not be combined with 'select'")
+          tags.delete("style")
+        end
+        tags
+      end
+
       def allowed_tags(options)
-        options[:tags] || self.class.allowed_tags
+        if options[:tags]
+          remove_safelist_tag_combinations(options[:tags])
+        else
+          self.class.allowed_tags
+        end
       end
 
       def allowed_attributes(options)

test/sanitizer_test.rb

@@ -581,6 +581,25 @@ def test_exclude_node_type_comment
     assert_equal("<div>text</div><b>text</b>", safe_list_sanitize("<div>text</div><!-- comment --><b>text</b>"))
   end
 
+  def test_disallow_the_dangerous_safelist_combination_of_select_and_style
+    input = "<select><style><script>alert(1)</script></style></select>"
+    tags = ["select", "style"]
+    warning = /WARNING: Rails::Html::SafeListSanitizer: removing 'style' from safelist/
+    sanitized = nil
+    invocation = Proc.new { sanitized = safe_list_sanitize(input, tags: tags) }
+
+    if html5_mode?
+      # if Loofah is using an HTML5 parser,
+      #   then "style" should be removed by the parser as an invalid child of "select"
+      assert_silent(&invocation)
+    else
+      # if Loofah is using an HTML4 parser,
+      #   then SafeListSanitizer should remove "style" from the safelist
+      assert_output(nil, warning, &invocation)
+    end
+    refute_includes(sanitized, "style")
+  end
+
 protected
 
   def xpath_sanitize(input, options = {})
@@ -641,4 +660,8 @@ def convert_to_css_hex(string, escape_parens=false)
   def libxml_2_9_14_recovery?
     Nokogiri.method(:uses_libxml?).arity == -1 && Nokogiri.uses_libxml?(">= 2.9.14")
   end
+
+  def html5_mode?
+    ::Loofah.respond_to?(:html5_mode?) && ::Loofah.html5_mode?
+  end
 end