Issue #2699489 by catch, tim.plunkett: FormBuilder $ajax_form_request check does not check which AJAX form is being requested

core/lib/Drupal/Core/Form/FormBuilder.php

@@ -316,7 +316,7 @@ class FormBuilder implements FormBuilderInterface, FormValidatorInterface, FormS
     // In case the post request exceeds the configured allowed size
     // (post_max_size), the post request is potentially broken. Add some
     // protection against that and at the same time have a nice error message.
-    if ($ajax_form_request && !isset($form_state->getUserInput()['form_id'])) {
+    if ($ajax_form_request && !$request->request->has('form_id')) {
       throw new BrokenPostRequestException($this->getFileUploadMaxSize());
     }
 
@@ -327,7 +327,9 @@ class FormBuilder implements FormBuilderInterface, FormValidatorInterface, FormS
     // then passed through
     // \Drupal\Core\Form\FormAjaxResponseBuilderInterface::buildResponse() to
     // build a proper AJAX response.
-    if ($ajax_form_request && $form_state->isProcessingInput()) {
+    // Only do this when the form ID matches, since there is no guarantee from
+    // $ajax_form_request that it's an AJAX request for this particular form.
+    if ($ajax_form_request && $form_state->isProcessingInput() && $request->request->get('form_id') == $form_id) {
       throw new FormAjaxException($form, $form_state);
     }
 

core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php

@@ -568,6 +568,29 @@ class FormBuilderTest extends FormTestBase {
     $this->formBuilder->buildForm($form_arg, $form_state);
   }
 
+  /**
+   * @covers ::buildForm
+   */
+  public function testGetPostAjaxRequest() {
+    $request = new Request([FormBuilderInterface::AJAX_FORM_REQUEST => TRUE], ['form_id' => 'different_form_id']);
+    $request->setMethod('POST');
+    $this->requestStack->push($request);
+
+    $form_state = (new FormState())
+      ->setUserInput([FormBuilderInterface::AJAX_FORM_REQUEST => TRUE])
+      ->setMethod('get')
+      ->setAlwaysProcess()
+      ->disableRedirect()
+      ->set('ajax', TRUE);
+
+    $form_id = '\Drupal\Tests\Core\Form\TestForm';
+    $expected_form = (new TestForm())->buildForm([], $form_state);
+
+    $form = $this->formBuilder->buildForm($form_id, $form_state);
+    $this->assertFormElement($expected_form, $form, 'test');
+    $this->assertSame('test-form', $form['#id']);
+  }
+
   /**
    * @covers ::buildForm
    *