statham_sanitizer.rb

module HTML
  class StathamSanitizer < WhiteListSanitizer

    protected

    def tokenize(text, options)
      super.map do |token|
        if token.is_a?(HTML::Tag) && options[:parent].include?(token.name)
          token.to_s.gsub(/</, "&lt;")
        else
          token
        end
      end
    end

    def process_node(node, result, options)
      result << case node
        when HTML::Tag
          if node.closing == :close && options[:parent].first == node.name
            options[:parent].shift
          elsif node.closing != :self
            options[:parent].unshift node.name
          end

          process_attributes_for node, options

          if options[:tags].include?(node.name)
            node
          else
            bad_tags.include?(node.name) ? nil : node.to_s.gsub(/</, "&lt;")
          end
        else
          bad_tags.include?(options[:parent].first) ? nil : node.to_s.gsub(/</, "&lt;")
      end
    end
  end
end

statham_sanitizer_test.rb

require File.dirname(__FILE__) + '/../test_helper'

class StathamSanitizerTest < ActiveSupport::TestCase
  context "A StathamSanitizer" do
    setup do
      @sanitizer = HTML::StathamSanitizer.new
    end

    should "escape tags that are neither allowed nor banned" do
      assert_equal "&lt;font>Hello&lt;/font>", @sanitizer.sanitize("<font>Hello</font>")
    end

    should "escape tags that are allowed but unclosed" do
      assert_equal "&lt;p>Hello", @sanitizer.sanitize("<p>Hello")
    end

    should "escape tags that are closed without ever being opened" do
      assert_equal "Hello&lt;/p>", @sanitizer.sanitize("Hello</p>")
    end

    should "include tags that are allowed and self-closing" do
      assert_equal "Hello<br />", @sanitizer.sanitize("Hello<br />")
    end

    should "escape comments" do
      assert_equal "&lt;!-- comment", @sanitizer.sanitize("<!-- comment")
    end
  end
end