Switch to on-by-default XSS escaping for rails.

This consists of:

  * String#html_safe! a method to mark a string as 'safe'
  * ActionView::SafeBuffer a string subclass which escapes anything unsafe which is concatenated to it
  * Calls to String#html_safe! throughout the rails helpers
  * a 'raw' helper which lets you concatenate trusted HTML from non-safety-aware sources (e.g. presantized strings in the DB)
  * New ERB implementation based on erubis which uses a SafeBuffer instead of a String

Hat tip to Django for the inspiration.

actionpack/Gemfile

@@ -4,6 +4,7 @@ gem "rack", "~> 1.0.0"
 gem "rack-test", "~> 0.5.0"
 gem "activesupport", "3.0.pre", :vendored_at => rails_root.join("activesupport")
 gem "activemodel", "3.0.pre", :vendored_at => rails_root.join("activemodel")
+gem "erubis", "~> 2.6.0"
 
 only :test do
   gem "mocha"

actionpack/lib/action_view.rb

@@ -44,11 +44,11 @@ module ActionView
   autoload :TextTemplate,      'action_view/template/text'
   autoload :Helpers,           'action_view/helpers'
   autoload :FileSystemResolverWithFallback, 'action_view/template/resolver'
+  autoload :SafeBuffer,        'action_view/safe_buffer'
 end
 
-class ERB
-  autoload :Util, 'action_view/erb/util'
-end
+require 'action_view/erb/util'
+
 
 I18n.load_path << "#{File.dirname(__FILE__)}/action_view/locale/en.yml"
 

actionpack/lib/action_view/base.rb

@@ -260,7 +260,7 @@ module ActionView #:nodoc:
       @assigns = assigns_for_first_render.each { |key, value| instance_variable_set("@#{key}", value) }
       @controller = controller
       @helpers = self.class.helpers || Module.new
-      @_content_for = Hash.new {|h,k| h[k] = "" }
+      @_content_for = Hash.new {|h,k| h[k] = ActionView::SafeBuffer.new }
       self.view_paths = view_paths
     end
 

actionpack/lib/action_view/erb/util.rb

@@ -15,9 +15,19 @@ class ERB
     #   puts html_escape("is a > 0 & a < 10?")
     #   # => is a &gt; 0 &amp; a &lt; 10?
     def html_escape(s)
-      s.to_s.gsub(/[&"><]/) { |special| HTML_ESCAPE[special] }
+      s = s.to_s
+      if s.html_safe?
+        s
+      else
+        s.gsub(/[&"><]/) { |special| HTML_ESCAPE[special] }.html_safe!
+      end
     end
 
+    alias h html_escape
+
+    module_function :html_escape
+    module_function :h
+
     # A utility method for escaping HTML entities in JSON strings.
     # This method is also aliased as <tt>j</tt>.
     #

actionpack/lib/action_view/helpers.rb

@@ -15,6 +15,7 @@ module ActionView #:nodoc:
     autoload :JavaScriptHelper, 'action_view/helpers/javascript_helper'
     autoload :NumberHelper, 'action_view/helpers/number_helper'
     autoload :PrototypeHelper, 'action_view/helpers/prototype_helper'
+    autoload :RawOutputHelper, 'action_view/helpers/raw_output_helper'
     autoload :RecordIdentificationHelper, 'action_view/helpers/record_identification_helper'
     autoload :RecordTagHelper, 'action_view/helpers/record_tag_helper'
     autoload :SanitizeHelper, 'action_view/helpers/sanitize_helper'
@@ -46,6 +47,7 @@ module ActionView #:nodoc:
     include JavaScriptHelper
     include NumberHelper
     include PrototypeHelper
+    include RawOutputHelper
     include RecordIdentificationHelper
     include RecordTagHelper
     include SanitizeHelper

actionpack/lib/action_view/helpers/active_model_helper.rb

@@ -91,6 +91,7 @@ module ActionView
         yield contents if block_given?
         contents << submit_tag(submit_value)
         contents << '</form>'
+        contents.html_safe!
       end
 
       # Returns a string containing the error message attached to the +method+ on the +object+ if one exists.

actionpack/lib/action_view/helpers/asset_tag_helper.rb

@@ -289,7 +289,7 @@ module ActionView
         else
           sources = expand_javascript_sources(sources, recursive)
           ensure_javascript_sources!(sources) if cache
-          sources.collect { |source| javascript_src_tag(source, options) }.join("\n")
+          sources.collect { |source| javascript_src_tag(source, options) }.join("\n").html_safe!
         end
       end
 
@@ -440,7 +440,7 @@ module ActionView
         else
           sources = expand_stylesheet_sources(sources, recursive)
           ensure_stylesheet_sources!(sources) if cache
-          sources.collect { |source| stylesheet_tag(source, options) }.join("\n")
+          sources.collect { |source| stylesheet_tag(source, options) }.join("\n").html_safe!
         end
       end
 
@@ -584,7 +584,7 @@ module ActionView
 
         if sources.is_a?(Array)
           content_tag("video", options) do
-            sources.map { |source| tag("source", :src => source) }.join
+            sources.map { |source| tag("source", :src => source) }.join.html_safe!
           end
         else
           options[:src] = path_to_video(sources)

actionpack/lib/action_view/helpers/capture_helper.rb

@@ -143,7 +143,7 @@ module ActionView
       # Defaults to a new empty string.
       def with_output_buffer(buf = nil) #:nodoc:
         unless buf
-          buf = ''
+          buf = ActionView::SafeBuffer.new
           buf.force_encoding(output_buffer.encoding) if buf.respond_to?(:force_encoding)
         end
         self.output_buffer, old_buffer = buf, output_buffer

actionpack/lib/action_view/helpers/date_helper.rb

@@ -916,15 +916,15 @@ module ActionView
 
     class InstanceTag #:nodoc:
       def to_date_select_tag(options = {}, html_options = {})
-        datetime_selector(options, html_options).select_date
+        datetime_selector(options, html_options).select_date.html_safe!
       end
 
       def to_time_select_tag(options = {}, html_options = {})
-        datetime_selector(options, html_options).select_time
+        datetime_selector(options, html_options).select_time.html_safe!
       end
 
       def to_datetime_select_tag(options = {}, html_options = {})
-        datetime_selector(options, html_options).select_datetime
+        datetime_selector(options, html_options).select_datetime.html_safe!
       end
 
       private

actionpack/lib/action_view/helpers/form_helper.rb

@@ -282,7 +282,7 @@ module ActionView
 
         concat(form_tag(options.delete(:url) || {}, options.delete(:html) || {}))
         fields_for(object_name, *(args << options), &proc)
-        concat('</form>')
+        concat('</form>'.html_safe!)
       end
 
       def apply_form_for_options!(object_or_array, options) #:nodoc:
@@ -809,7 +809,7 @@ module ActionView
         add_default_name_and_id(options)
         hidden = tag("input", "name" => options["name"], "type" => "hidden", "value" => options['disabled'] && checked ? checked_value : unchecked_value)
         checkbox = tag("input", options)
-        hidden + checkbox
+        (hidden + checkbox).html_safe!
       end
 
       def to_boolean_select_tag(options = {})

actionpack/lib/action_view/helpers/form_options_helper.rb

@@ -296,7 +296,7 @@ module ActionView
           options << %(<option value="#{html_escape(value.to_s)}"#{selected_attribute}#{disabled_attribute}>#{html_escape(text.to_s)}</option>)
         end
 
-        options_for_select.join("\n")
+        options_for_select.join("\n").html_safe!
       end
 
       # Returns a string of option tags that have been compiled by iterating over the +collection+ and assigning the

actionpack/lib/action_view/helpers/form_tag_helper.rb

@@ -440,7 +440,7 @@ module ActionView
         concat(tag(:fieldset, options, true))
         concat(content_tag(:legend, legend)) unless legend.blank?
         concat(content)
-        concat("</fieldset>")
+        concat("</fieldset>".html_safe!)
       end
 
       private
@@ -467,14 +467,14 @@ module ActionView
 
         def form_tag_html(html_options)
           extra_tags = extra_tags_for_form(html_options)
-          tag(:form, html_options, true) + extra_tags
+          (tag(:form, html_options, true) + extra_tags).html_safe!
         end
 
         def form_tag_in_block(html_options, &block)
           content = capture(&block)
           concat(form_tag_html(html_options))
           concat(content)
-          concat("</form>")
+          concat("</form>".html_safe!)
         end
 
         def token_tag

actionpack/lib/action_view/helpers/prototype_helper.rb

@@ -395,7 +395,7 @@ module ActionView
 
         concat(form_remote_tag(options))
         fields_for(object_name, *(args << options), &proc)
-        concat('</form>')
+        concat('</form>'.html_safe!)
       end
       alias_method :form_remote_for, :remote_form_for
 

actionpack/lib/action_view/helpers/raw_output_helper.rb

@@ -0,0 +1,9 @@
+module ActionView #:nodoc:
+  module Helpers #:nodoc:
+    module RawOutputHelper
+      def raw(stringish)
+        stringish.to_s.html_safe!
+      end
+    end
+  end
+end

actionpack/lib/action_view/helpers/sanitize_helper.rb

@@ -49,7 +49,11 @@ module ActionView
       # confuse browsers.
       #
       def sanitize(html, options = {})
-        self.class.white_list_sanitizer.sanitize(html, options)
+        returning self.class.white_list_sanitizer.sanitize(html, options) do |sanitized|
+          if sanitized
+            sanitized.html_safe!
+          end
+        end
       end
 
       # Sanitizes a block of CSS code. Used by +sanitize+ when it comes across a style attribute.
@@ -72,7 +76,11 @@ module ActionView
       #   strip_tags("<div id='top-bar'>Welcome to my website!</div>")
       #   # => Welcome to my website!
       def strip_tags(html)
-        self.class.full_sanitizer.sanitize(html)
+        returning self.class.full_sanitizer.sanitize(html) do |sanitized|
+          if sanitized
+            sanitized.html_safe!
+          end
+        end
       end
 
       # Strips all link tags from +text+ leaving just the link text.

actionpack/lib/action_view/helpers/tag_helper.rb

@@ -41,7 +41,7 @@ module ActionView
       #   tag("img", { :src => "open & shut.png" }, false, false)
       #   # => <img src="open &amp; shut.png" />
       def tag(name, options = nil, open = false, escape = true)
-        "<#{name}#{tag_options(options, escape) if options}#{open ? ">" : " />"}"
+        "<#{name}#{tag_options(options, escape) if options}#{open ? ">" : " />"}".html_safe!
       end
 
       # Returns an HTML block tag of type +name+ surrounding the +content+. Add
@@ -94,7 +94,7 @@ module ActionView
       #   cdata_section(File.read("hello_world.txt"))
       #   # => <![CDATA[<hello from a text file]]>
       def cdata_section(content)
-        "<![CDATA[#{content}]]>"
+        "<![CDATA[#{content}]]>".html_safe!
       end
 
       # Returns an escaped version of +html+ without affecting existing escaped entities.
@@ -128,7 +128,7 @@ module ActionView
 
         def content_tag_string(name, content, options, escape = true)
           tag_options = tag_options(options, escape) if options
-          "<#{name}#{tag_options}>#{content}</#{name}>"
+          "<#{name}#{tag_options}>#{content}</#{name}>".html_safe!
         end
 
         def tag_options(options, escape = true)
@@ -143,7 +143,7 @@ module ActionView
                 attrs << %(#{key}="#{final_value}")
               end
             end
-            " #{attrs.sort * ' '}" unless attrs.empty?
+            " #{attrs.sort * ' '}".html_safe! unless attrs.empty?
           end
         end
     end

actionpack/lib/action_view/helpers/url_helper.rb

@@ -93,7 +93,7 @@ module ActionView
           polymorphic_path(options)
         end
 
-        escape ? escape_once(url) : url
+        (escape ? escape_once(url) : url).html_safe!
       end
 
       # Creates a link tag of the given +name+ using a URL created by the set
@@ -220,7 +220,7 @@ module ActionView
         if block_given?
           options      = args.first || {}
           html_options = args.second
-          concat(link_to(capture(&block), options, html_options))
+          concat(link_to(capture(&block), options, html_options).html_safe!)
         else
           name         = args[0]
           options      = args[1] || {}
@@ -238,7 +238,7 @@ module ActionView
           end
 
           href_attr = "href=\"#{url}\"" unless href
-          "<a #{href_attr}#{tag_options}>#{name || url}</a>"
+          "<a #{href_attr}#{tag_options}>#{ERB::Util.h(name || url)}</a>".html_safe!
         end
       end
 
@@ -309,8 +309,8 @@ module ActionView
 
         html_options.merge!("type" => "submit", "value" => name)
 
-        "<form method=\"#{form_method}\" action=\"#{escape_once url}\" class=\"button-to\"><div>" +
-          method_tag + tag("input", html_options) + request_token_tag + "</div></form>"
+        ("<form method=\"#{form_method}\" action=\"#{escape_once url}\" class=\"button-to\"><div>" +
+          method_tag + tag("input", html_options) + request_token_tag + "</div></form>").html_safe!
       end
 
 

actionpack/lib/action_view/render/partials.rb

@@ -223,7 +223,7 @@ module ActionView
         end
 
         result = template ? collection_with_template(template) : collection_without_template
-        result.join(spacer)
+        result.join(spacer).html_safe!
       end
 
       def collection_with_template(template)

actionpack/lib/action_view/safe_buffer.rb

@@ -0,0 +1,28 @@
+
+module ActionView #:nodoc:
+  class SafeBuffer < String
+    def <<(value)
+      if value.html_safe?
+        super(value)
+      else
+        super(CGI.escapeHTML(value))
+      end
+    end
+
+    def concat(value)
+      self << value
+    end
+
+    def html_safe?
+      true
+    end
+
+    def html_safe!
+      self
+    end
+
+    def to_s
+      self
+    end
+  end
+end

actionpack/lib/action_view/template/handlers/erb.rb

@@ -1,7 +1,31 @@
 require 'active_support/core_ext/class/attribute_accessors'
+require 'active_support/core_ext/string/output_safety'
+require 'erubis'
 
 module ActionView
   module TemplateHandlers
+    class Erubis < ::Erubis::Eruby
+      def add_preamble(src)
+        src << "@output_buffer = ActionView::SafeBuffer.new;"
+      end
+
+      def add_text(src, text)
+        src << "@output_buffer << ('" << escape_text(text) << "'.html_safe!);"
+      end
+
+      def add_expr_literal(src, code)
+        src << '@output_buffer << ((' << code << ').to_s);'
+      end
+
+      def add_expr_escaped(src, code)
+        src << '@output_buffer << ' << escaped_expr(code) << ';'
+      end
+
+      def add_postamble(src)
+        src << '@output_buffer.to_s'
+      end
+    end
+
     class ERB < TemplateHandler
       include Compilable
 
@@ -15,11 +39,9 @@ module ActionView
       self.default_format = Mime::HTML
 
       def compile(template)
-        require 'erb'
-
         magic = $1 if template.source =~ /\A(<%#.*coding[:=]\s*(\S+)\s*-?%>)/
         erb = "#{magic}<% __in_erb_template=true %>#{template.source}"
-        ::ERB.new(erb, nil, erb_trim_mode, '@output_buffer').src
+        Erubis.new(erb, :trim=>(self.class.erb_trim_mode == "-")).src
       end
     end
   end

actionpack/lib/action_view/test_case.rb

@@ -55,7 +55,7 @@ module ActionView
     setup :setup_with_controller
     def setup_with_controller
       @controller = TestController.new
-      @output_buffer = ''
+      @output_buffer = ActionView::SafeBuffer.new
       @rendered = ''
 
       self.class.send(:include_helper_modules!)

actionpack/test/controller/output_escaping_test.rb

@@ -0,0 +1,19 @@
+require 'abstract_unit'
+
+class OutputEscapingTest < ActiveSupport::TestCase
+
+  test "escape_html shouldn't die when passed nil" do
+    assert ERB::Util.h(nil).blank?
+  end
+
+  test "escapeHTML should escape strings" do
+    assert_equal "&lt;&gt;&quot;", ERB::Util.h("<>\"")
+  end
+
+  test "escapeHTML shouldn't touch explicitly safe strings" do
+    # TODO this seems easier to compose and reason about, but
+    # this should be verified
+    assert_equal "<", ERB::Util.h("<".html_safe!)
+  end
+
+end

actionpack/test/controller/render_test.rb

@@ -1050,7 +1050,7 @@ class RenderTest < ActionController::TestCase
 
   def test_action_talk_to_layout
     get :action_talk_to_layout
-    assert_equal "<title>Talking to the layout</title>\nAction was here!", @response.body
+    assert_equal "<title>Talking to the layout</title>\n\nAction was here!", @response.body
   end
 
   # :addressed:

actionpack/test/template/asset_tag_helper_test.rb

@@ -231,6 +231,11 @@ class AssetTagHelperTest < ActionView::TestCase
     assert_dom_equal(%(<script src="/javascripts/prototype.js?1" type="text/javascript"></script>\n<script src="/javascripts/effects.js?1" type="text/javascript"></script>\n<script src="/javascripts/dragdrop.js?1" type="text/javascript"></script>\n<script src="/javascripts/controls.js?1" type="text/javascript"></script>\n<script src="/javascripts/application.js?1" type="text/javascript"></script>), javascript_include_tag(:defaults))
   end
 
+  def test_javascript_include_tag_is_html_safe
+    assert javascript_include_tag(:defaults).html_safe?
+    assert javascript_include_tag("prototype").html_safe?
+  end
+
   def test_register_javascript_include_default
     ENV["RAILS_ASSET_ID"] = ""
     ActionView::Helpers::AssetTagHelper::register_javascript_include_default 'bank'
@@ -285,6 +290,13 @@ class AssetTagHelperTest < ActionView::TestCase
     }
   end
 
+  def test_stylesheet_link_tag_is_html_safe
+    ENV["RAILS_ASSET_ID"] = ""
+    assert stylesheet_link_tag('dir/file').html_safe?
+    assert stylesheet_link_tag('dir/other/file', 'dir/file2').html_safe?
+    assert stylesheet_tag('dir/file', {}).html_safe?
+  end
+
   def test_custom_stylesheet_expansions
     ENV["RAILS_ASSET_ID"] = ''
     ActionView::Helpers::AssetTagHelper::register_stylesheet_expansion :robbery => ["bank", "robber"]

actionpack/test/template/erb_util_test.rb

@@ -15,6 +15,18 @@ class ErbUtilTest < Test::Unit::TestCase
     end
   end
   
+  def test_html_escape_is_html_safe
+    escaped = h("<p>")
+    assert_equal "&lt;p&gt;", escaped
+    assert escaped.html_safe?
+  end
+
+  def test_html_escape_passes_html_escpe_unmodified
+    escaped = h("<p>".html_safe!)
+    assert_equal "<p>", escaped
+    assert escaped.html_safe?
+  end
+
   def test_rest_in_ascii
     (0..127).to_a.map {|int| int.chr }.each do |chr|
       next if %w(& " < >).include?(chr)

actionpack/test/template/form_helper_test.rb

@@ -974,7 +974,7 @@ class FormHelperTest < ActionView::TestCase
     (field_helpers - %w(hidden_field)).each do |selector|
       src = <<-END_SRC
         def #{selector}(field, *args, &proc)
-          "<label for='\#{field}'>\#{field.to_s.humanize}:</label> " + super + "<br/>"
+          ("<label for='\#{field}'>\#{field.to_s.humanize}:</label> " + super + "<br/>").html_safe!
         end
       END_SRC
       class_eval src, __FILE__, __LINE__

actionpack/test/template/raw_output_helper_test.rb

@@ -0,0 +1,21 @@
+require 'abstract_unit'
+require 'testing_sandbox'
+
+class RawOutputHelperTest < ActionView::TestCase
+  tests ActionView::Helpers::RawOutputHelper
+  include TestingSandbox
+
+  def setup
+    @string = "hello"
+  end
+
+  test "raw returns the safe string" do
+    result = raw(@string)
+    assert_equal @string, result
+    assert result.html_safe?
+  end
+
+  test "raw handles nil values correctly" do
+    assert_equal "", raw(nil)
+  end
+end

actionpack/test/template/render_test.rb

@@ -229,7 +229,7 @@ module RenderTestCases
   end
 
   def test_render_with_nested_layout
-    assert_equal %(<title>title</title>\n<div id="column">column</div>\n<div id="content">content</div>\n),
+    assert_equal %(<title>title</title>\n\n\n<div id="column">column</div>\n<div id="content">content</div>\n),
       @view.render(:file => "test/nested_layout.erb", :layout => "layouts/yield")
   end
 

actionpack/test/template/sanitize_helper_test.rb

@@ -39,7 +39,16 @@ class SanitizeHelperTest < ActionView::TestCase
     %{This is a test.\n\n\nIt no longer contains any HTML.\n}, strip_tags(
     %{<title>This is <b>a <a href="" target="_blank">test</a></b>.</title>\n\n<!-- it has a comment -->\n\n<p>It no <b>longer <strong>contains <em>any <strike>HTML</strike></em>.</strong></b></p>\n}))
     assert_equal "This has a  here.", strip_tags("This has a <!-- comment --> here.")
-    [nil, '', '   '].each { |blank| assert_equal blank, strip_tags(blank) }
+    [nil, '', '   '].each do |blank|
+      stripped = strip_tags(blank)
+      assert_equal blank, stripped
+      assert stripped.html_safe? unless blank.nil?
+    end
+    assert strip_tags("<script>").html_safe?
+  end
+
+  def test_sanitize_is_marked_safe
+    assert sanitize("<html><script></script></html>").html_safe?
   end
 
   def assert_sanitized(text, expected = nil)

actionpack/test/template/tag_helper_test.rb

@@ -34,6 +34,7 @@ class TagHelperTest < ActionView::TestCase
 
   def test_content_tag
     assert_equal "<a href=\"create\">Create</a>", content_tag("a", "Create", "href" => "create")
+    assert content_tag("a", "Create", "href" => "create").html_safe?
     assert_equal content_tag("a", "Create", "href" => "create"),
                  content_tag("a", "Create", :href => "create")
   end

actionpack/test/template/test_case_test.rb

@@ -155,7 +155,7 @@ module ActionView
     class AssertionsTest < ActionView::TestCase
       def render_from_helper
         form_tag('/foo') do
-          concat render(:text => '<ul><li>foo</li></ul>')
+          concat render(:text => '<ul><li>foo</li></ul>').html_safe!
         end
       end
       helper_method :render_from_helper

actionpack/test/template/url_helper_test.rb

@@ -139,7 +139,7 @@ class UrlHelperTest < ActionView::TestCase
   end
 
   def test_link_tag_with_img
-    assert_dom_equal "<a href=\"http://www.example.com\"><img src='/favicon.jpg' /></a>", link_to("<img src='/favicon.jpg' />", "http://www.example.com")
+    assert_dom_equal "<a href=\"http://www.example.com\"><img src='/favicon.jpg' alt=\"Favicon\"  /></a>", link_to(image_tag("/favicon.jpg"), "http://www.example.com")
   end
 
   def test_link_with_nil_html_options

actionpack/test/view/safe_buffer_test.rb

@@ -0,0 +1,41 @@
+require 'abstract_unit'
+
+class SafeBufferTest < ActionView::TestCase
+  def setup
+    @buffer = ActionView::SafeBuffer.new
+  end
+
+  test "Should look like a string" do
+    assert @buffer.is_a?(String)
+    assert_equal "", @buffer
+  end
+
+  test "Should escape a raw string which is passed to them" do
+    @buffer << "<script>"
+    assert_equal "&lt;script&gt;", @buffer
+  end
+
+  test "Should NOT escape a safe value passed to it" do
+    @buffer << "<script>".html_safe!
+    assert_equal "<script>", @buffer
+  end
+
+  test "Should not mess with an innocuous string" do
+    @buffer << "Hello"
+    assert_equal "Hello", @buffer
+  end
+
+  test "Should not mess with a previously escape test" do
+    @buffer << CGI.escapeHTML("<script>")
+    assert_equal "&lt;script&gt;", @buffer
+  end
+
+  test "Should be considered safe" do
+    assert @buffer.html_safe?
+  end
+
+  test "Should return a safe buffer when calling to_s" do
+    new_buffer = @buffer.to_s
+    assert_equal ActionView::SafeBuffer, new_buffer.class
+  end
+end

activesupport/lib/active_support/core_ext/string.rb

@@ -7,4 +7,5 @@ require 'active_support/core_ext/string/access'
 require 'active_support/core_ext/string/iterators'
 require 'active_support/core_ext/string/xchar'
 require 'active_support/core_ext/string/behavior'
-require 'active_support/core_ext/string/interpolation'
+require 'active_support/core_ext/string/interpolation'
+require 'active_support/core_ext/string/output_safety'

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -0,0 +1,43 @@
+class String
+  def html_safe?
+    defined?(@_rails_html_safe) && @_rails_html_safe
+  end
+
+  def html_safe!
+    @_rails_html_safe = true
+    self
+  end
+  
+  def html_safe
+    dup.html_safe!
+  end
+  
+  alias original_plus +
+  def +(other)
+    result = original_plus(other)
+    if html_safe? && also_html_safe?(other)
+      result.html_safe!
+    else
+      result
+    end
+  end
+  
+  alias original_concat <<
+  def <<(other)
+    result = original_concat(other)
+    unless html_safe? && also_html_safe?(other)
+      @_rails_html_safe = false
+    end
+    result
+  end
+  
+  def concat(other)
+    self << other
+  end
+  
+  private
+    def also_html_safe?(other)
+      other.respond_to?(:html_safe?) && other.html_safe?
+    end
+  
+end

activesupport/test/core_ext/string_ext_test.rb

@@ -356,3 +356,89 @@ class StringBytesizeTest < Test::Unit::TestCase
     assert_equal 3, 'foo'.bytesize
   end
 end
+
+class OutputSafetyTest < ActiveSupport::TestCase
+  def setup
+    @string = "hello"
+  end
+
+  test "A string is unsafe by default" do
+    assert !@string.html_safe?
+  end
+
+  test "A string can be marked safe" do
+    @string.html_safe!
+    assert @string.html_safe?
+  end
+
+  test "Marking a string safe returns the string" do
+    assert_equal @string, @string.html_safe!
+  end
+
+  test "Adding a safe string to another safe string returns a safe string" do
+    @other_string = "other".html_safe!
+    @string.html_safe!
+    @combination = @other_string + @string
+
+    assert_equal "otherhello", @combination
+    assert @combination.html_safe?
+  end
+
+  test "Adding an unsafe string to a safe string returns an unsafe string" do
+    @other_string = "other".html_safe!
+    @combination = @other_string + @string
+    @other_combination = @string + @other_string
+
+    assert_equal "otherhello", @combination
+    assert_equal "helloother", @other_combination
+
+    assert !@combination.html_safe?
+    assert !@other_combination.html_safe?
+  end
+
+  test "Concatting safe onto unsafe yields unsafe" do
+    @other_string = "other"
+    @string.html_safe!
+
+    @other_string.concat(@string)
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting unsafe onto safe yields unsafe" do
+    @other_string = "other".html_safe!
+
+    @other_string.concat(@string)
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting safe onto safe yields safe" do
+    @other_string = "other".html_safe!
+    @string.html_safe!
+
+    @other_string.concat(@string)
+    assert @other_string.html_safe?
+  end
+
+  test "Concatting safe onto unsafe with << yields unsafe" do
+    @other_string = "other"
+    @string.html_safe!
+
+    @other_string << @string
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting unsafe onto safe with << yields unsafe" do
+    @other_string = "other".html_safe!
+
+    @other_string << @string
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting safe onto safe with << yields safe" do
+    @other_string = "other".html_safe!
+    @string.html_safe!
+
+    @other_string << @string
+    assert @other_string.html_safe?
+  end
+end