core/lib/Drupal/Core/Form/FormBuilder.php          | 12 ++++++++++
 .../Drupal/Core/Render/Element/RenderElement.php   |  2 +-
 .../system/src/Controller/FormAjaxController.php   | 27 +++++++++++++++++++---
 3 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 4b995c1..fa411a2 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -17,6 +17,7 @@
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Render\ElementInfoManagerInterface;
+use Drupal\Core\Site\Settings;
 use Drupal\Core\Theme\ThemeManagerInterface;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
@@ -180,6 +181,10 @@ public function getForm($form_arg) {
    */
   public function buildForm($form_id, FormStateInterface &$form_state) {
     // Ensure the form ID is prepared.
+    $private_key = \Drupal::service('private_key');
+    $keyvalue = \Drupal::service('keyvalue.expirable');
+    $form_build_key = 'form_build_key:' . Crypt::hmacBase64(serialize($form_id) . serialize($form_state), 'form_key' . $private_key->get() . Settings::getHashSalt());
+    $keyvalue->get('form_build_key')->setWithExpire($form_build_key, [$form_id, $form_state], 21600);
     $form_id = $this->getFormId($form_id, $form_state);
 
     $input = $form_state->getUserInput();
@@ -221,6 +226,13 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
       $form = $this->retrieveForm($form_id, $form_state);
       $this->prepareForm($form_id, $form, $form_state);
 
+      $form['form_build_key'] = array(
+        '#type' => 'hidden',
+        '#value' => $form_build_key,
+        '#name' => 'form_build_key',
+        '#parents' => array('form_build_key'),
+      );
+
       // self::setCache() removes uncacheable $form_state keys (see properties
       // in \Drupal\Core\Form\FormState) in order for multi-step forms to work
       // properly. This means that form processing logic for single-step forms
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index 0283333..b064782 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -128,7 +128,7 @@ public static function preRenderGroup($element) {
    */
   public static function processAjaxForm(&$element, FormStateInterface $form_state, &$complete_form) {
     $element = static::preRenderAjaxForm($element);
-    if (!empty($element['#ajax_processed'])) {
+    if (!empty($element['#ajax_processed']) && isset($element['#ajax']['url'])) {
       $form_state->setCached();
     }
     return $element;
diff --git a/core/modules/system/src/Controller/FormAjaxController.php b/core/modules/system/src/Controller/FormAjaxController.php
index 0c29322..87210fe 100644
--- a/core/modules/system/src/Controller/FormAjaxController.php
+++ b/core/modules/system/src/Controller/FormAjaxController.php
@@ -114,13 +114,11 @@ public static function create(ContainerInterface $container) {
    * @throws \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface
    */
   public function content(Request $request) {
-    $ajax_form = $this->getForm($request);
+    $ajax_form = $this->buildForm($request);
     $form = $ajax_form->getForm();
     $form_state = $ajax_form->getFormState();
     $commands = $ajax_form->getCommands();
 
-    $this->formBuilder->processForm($form['#form_id'], $form, $form_state);
-
     // We need to return the part of the form (or some other content) that needs
     // to be re-rendered so the browser can update the page with changed content.
     // Since this is the generic menu callback used by many Ajax elements, it is
@@ -216,4 +214,27 @@ protected function getForm(Request $request) {
     return new FileAjaxForm($form, $form_state, $form_id, $form_build_id, $commands);
   }
 
+  protected function buildForm(Request $request) {
+    $form_build_id = $request->request->get('form_build_id');
+    $form_build_key = $request->request->get('form_build_key');
+
+    $keyvalue = \Drupal::service('keyvalue.expirable');
+    $result = $keyvalue->get('form_build_key')->get($form_build_key);
+    if ($result === NULL) {
+      $this->logger->warning('Expired form.');
+      throw new BadRequestHttpException();
+    }
+    else {
+      list($form_id, $form_state) = $result;
+      $form = $this->formBuilder->buildForm($form_id, $form_state);
+
+      $commands = [];
+      if ($form_build_id != $form['#build_id']) {
+        // If the form build ID has changed, issue an Ajax command to update it.
+        $commands[] = new UpdateBuildIdCommand($form_build_id, $form['#build_id']);
+      }
+      return new FileAjaxForm($form, $form_state, $form_id, $form['#build_id'], $commands);
+    }
+  }
+
 }