diff --git a/core/lib/Drupal/Core/Entity/ContentEntityConfirmFormBase.php b/core/lib/Drupal/Core/Entity/ContentEntityConfirmFormBase.php
index 887fcae..a2874ce 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityConfirmFormBase.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityConfirmFormBase.php
@@ -121,7 +121,7 @@ public function delete(array $form, FormStateInterface $form_state) {}
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     // Override the default validation implementation as it is not necessary
     // nor possible to validate an entity in a confirmation form.
   }
diff --git a/core/lib/Drupal/Core/Entity/ContentEntityForm.php b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
index 3740600..81da645 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityForm.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
@@ -74,7 +74,7 @@ public function form(array $form, FormStateInterface $form_state) {
    * For more information about entity validation, see
    * https://www.drupal.org/node/2015613.
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     $entity = $this->buildEntity($form, $form_state);
     $this->getFormDisplay($form_state)->validateFormValues($entity, $form, $form_state);
 
diff --git a/core/lib/Drupal/Core/Entity/EntityForm.php b/core/lib/Drupal/Core/Entity/EntityForm.php
index fb015c7..60834c1 100644
--- a/core/lib/Drupal/Core/Entity/EntityForm.php
+++ b/core/lib/Drupal/Core/Entity/EntityForm.php
@@ -247,7 +247,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     // @todo Remove this.
     // Execute legacy global validation handlers.
     $form_state->setValidateHandlers([]);
diff --git a/core/lib/Drupal/Core/Entity/EntityFormInterface.php b/core/lib/Drupal/Core/Entity/EntityFormInterface.php
index 29b248b..d37f56c 100644
--- a/core/lib/Drupal/Core/Entity/EntityFormInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityFormInterface.php
@@ -105,7 +105,7 @@ public function buildEntity(array $form, FormStateInterface $form_state);
    * @return \Drupal\Core\Entity\ContentEntityTypeInterface
    *   The built entity.
    */
-  public function validate(array $form, FormStateInterface $form_state);
+  public function validate(array &$form, FormStateInterface $form_state);
 
   /**
    * Form submission handler for the 'save' action.
diff --git a/core/modules/action/src/ActionFormBase.php b/core/modules/action/src/ActionFormBase.php
index 96bb63d..864a067 100644
--- a/core/modules/action/src/ActionFormBase.php
+++ b/core/modules/action/src/ActionFormBase.php
@@ -123,7 +123,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     if ($this->plugin instanceof PluginFormInterface) {
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index 08d7508..5746e31 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -273,7 +273,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // The Block Entity form puts all block plugin form elements in the
diff --git a/core/modules/block_content/src/BlockContentForm.php b/core/modules/block_content/src/BlockContentForm.php
index 379e576..857776b 100644
--- a/core/modules/block_content/src/BlockContentForm.php
+++ b/core/modules/block_content/src/BlockContentForm.php
@@ -223,7 +223,7 @@ public function save(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validateForm(array &$form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     if ($this->entity->isNew()) {
       $exists = $this->blockContentStorage->loadByProperties(array('info' => $form_state->getValue(['info', 0, 'value'])));
       if (!empty($exists)) {
diff --git a/core/modules/comment/src/CommentForm.php b/core/modules/comment/src/CommentForm.php
index 10c2152..7a8cf50 100644
--- a/core/modules/comment/src/CommentForm.php
+++ b/core/modules/comment/src/CommentForm.php
@@ -314,7 +314,7 @@ public function buildEntity(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     $comment = parent::validate($form, $form_state);
 
     // Customly trigger validation of manually added fields and add in
diff --git a/core/modules/contact/src/ContactFormEditForm.php b/core/modules/contact/src/ContactFormEditForm.php
index 3300c59..6b80e0e 100644
--- a/core/modules/contact/src/ContactFormEditForm.php
+++ b/core/modules/contact/src/ContactFormEditForm.php
@@ -112,7 +112,7 @@ public function form(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // Validate and each email recipient.
diff --git a/core/modules/contact/src/MessageForm.php b/core/modules/contact/src/MessageForm.php
index 78de6dd..aeeb2cc 100644
--- a/core/modules/contact/src/MessageForm.php
+++ b/core/modules/contact/src/MessageForm.php
@@ -187,7 +187,7 @@ public function preview(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     $message = $this->entity;
diff --git a/core/modules/field_ui/src/Form/EntityDisplayModeAddForm.php b/core/modules/field_ui/src/Form/EntityDisplayModeAddForm.php
index d6d801b..4c83b50 100644
--- a/core/modules/field_ui/src/Form/EntityDisplayModeAddForm.php
+++ b/core/modules/field_ui/src/Form/EntityDisplayModeAddForm.php
@@ -38,7 +38,7 @@ public function buildForm(array $form, FormStateInterface $form_state, $entity_t
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     $form_state->setValueForElement($form['id'], $this->targetEntityTypeId . '.' . $form_state->getValue('id'));
diff --git a/core/modules/field_ui/src/Form/FieldConfigEditForm.php b/core/modules/field_ui/src/Form/FieldConfigEditForm.php
index 8a3cd54..27075d3 100644
--- a/core/modules/field_ui/src/Form/FieldConfigEditForm.php
+++ b/core/modules/field_ui/src/Form/FieldConfigEditForm.php
@@ -150,7 +150,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     if (isset($form['default_value'])) {
diff --git a/core/modules/field_ui/src/Form/FieldStorageConfigEditForm.php b/core/modules/field_ui/src/Form/FieldStorageConfigEditForm.php
index 58c4ea9..f49e747 100644
--- a/core/modules/field_ui/src/Form/FieldStorageConfigEditForm.php
+++ b/core/modules/field_ui/src/Form/FieldStorageConfigEditForm.php
@@ -146,7 +146,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // Validate field cardinality.
diff --git a/core/modules/filter/src/FilterFormatFormBase.php b/core/modules/filter/src/FilterFormatFormBase.php
index cbc68d6..fb82b3b 100644
--- a/core/modules/filter/src/FilterFormatFormBase.php
+++ b/core/modules/filter/src/FilterFormatFormBase.php
@@ -204,7 +204,7 @@ public function exists($format_id) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // @todo Move trimming upstream.
diff --git a/core/modules/node/src/NodeForm.php b/core/modules/node/src/NodeForm.php
index 44fccbf..5d34b1a 100644
--- a/core/modules/node/src/NodeForm.php
+++ b/core/modules/node/src/NodeForm.php
@@ -286,7 +286,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     $node = parent::validate($form, $form_state);
 
     if ($node->id() && (node_last_changed($node->id(), $this->getFormLangcode($form_state)) > $node->getChangedTime())) {
diff --git a/core/modules/node/src/NodeTypeForm.php b/core/modules/node/src/NodeTypeForm.php
index 6dfc669..f7307d6 100644
--- a/core/modules/node/src/NodeTypeForm.php
+++ b/core/modules/node/src/NodeTypeForm.php
@@ -204,7 +204,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     $id = trim($form_state->getValue('type'));
diff --git a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
index 365f7a5..ec6992f 100644
--- a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
+++ b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
@@ -133,7 +133,7 @@ public function form(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     // Only validate on edit.
     if ($form_state->hasValue('keyed_styles')) {
       // Check if another breakpoint group is selected.
diff --git a/core/modules/search/src/Form/SearchPageFormBase.php b/core/modules/search/src/Form/SearchPageFormBase.php
index 95afb4b..02e95ca 100644
--- a/core/modules/search/src/Form/SearchPageFormBase.php
+++ b/core/modules/search/src/Form/SearchPageFormBase.php
@@ -144,7 +144,7 @@ public function exists($id) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // Ensure each path is unique.
diff --git a/core/modules/system/src/Form/DateFormatFormBase.php b/core/modules/system/src/Form/DateFormatFormBase.php
index c843ba2..bece243 100644
--- a/core/modules/system/src/Form/DateFormatFormBase.php
+++ b/core/modules/system/src/Form/DateFormatFormBase.php
@@ -155,7 +155,7 @@ public function form(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // The machine name field should already check to see if the requested
diff --git a/core/modules/taxonomy/src/TermForm.php b/core/modules/taxonomy/src/TermForm.php
index daa2868..b7116cb 100644
--- a/core/modules/taxonomy/src/TermForm.php
+++ b/core/modules/taxonomy/src/TermForm.php
@@ -94,7 +94,7 @@ public function form(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     // Ensure numeric values.
diff --git a/core/modules/user/src/AccountForm.php b/core/modules/user/src/AccountForm.php
index 16ab69f..81e2ee6 100644
--- a/core/modules/user/src/AccountForm.php
+++ b/core/modules/user/src/AccountForm.php
@@ -354,7 +354,7 @@ public function buildEntity(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     /** @var \Drupal\user\UserInterface $account */
     $account = parent::validate($form, $form_state);
 
diff --git a/core/modules/views_ui/src/ViewAddForm.php b/core/modules/views_ui/src/ViewAddForm.php
index 9750094..91dcf50 100644
--- a/core/modules/views_ui/src/ViewAddForm.php
+++ b/core/modules/views_ui/src/ViewAddForm.php
@@ -162,7 +162,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     $wizard_type = $form_state->getValue(array('show', 'wizard_key'));
     $wizard_instance = $this->wizardManager->createInstance($wizard_type);
     $form_state->set('wizard', $wizard_instance->getPluginDefinition());
diff --git a/core/modules/views_ui/src/ViewEditForm.php b/core/modules/views_ui/src/ViewEditForm.php
index 2fc7fc4..b27233b 100644
--- a/core/modules/views_ui/src/ViewEditForm.php
+++ b/core/modules/views_ui/src/ViewEditForm.php
@@ -258,7 +258,7 @@ protected function actions(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function validate(array $form, FormStateInterface $form_state) {
+  public function validate(array &$form, FormStateInterface $form_state) {
     parent::validate($form, $form_state);
 
     $view = $this->entity;
diff --git a/core/tests/Drupal/Tests/Core/Entity/EntityFormTest.php b/core/tests/Drupal/Tests/Core/Entity/EntityFormTest.php
index 810bdd1..7d34fc1 100644
--- a/core/tests/Drupal/Tests/Core/Entity/EntityFormTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/EntityFormTest.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\Entity\EntityForm;
 use Drupal\Core\Form\FormState;
+use Drupal\Core\Form\FormStateInterface;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -123,4 +124,20 @@ public function testCopyFormValuesToEntity() {
     $this->assertNull($result->get('key_controlled_by_plugin_collection'));
   }
 
+  /**
+   * @covers ::validate
+   */
+  public function testValidate() {
+    $form_object = $this->getMock('Drupal\Core\Entity\EntityFormInterface');
+    $form_object->expects($this->once())
+      ->method('validate')
+      ->willReturnCallback(function (array &$form, FormStateInterface $form_state) {
+        $form['foo'] = 'bar';
+      });
+    $form['baz'] = 'bim';
+    $form_state = new FormState();
+    $form_object->validate($form, $form_state);
+    $this->assertArrayHasKey('foo', $form);
+  }
+
 }