diff --git a/core/modules/comment/src/CommentForm.php b/core/modules/comment/src/CommentForm.php
index 15d1ece..90987f0 100644
--- a/core/modules/comment/src/CommentForm.php
+++ b/core/modules/comment/src/CommentForm.php
@@ -323,12 +323,33 @@ protected function getEditedFieldNames(FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    /* @var \Drupal\Core\Entity\ContentEntityInterface $entity */
+    $entity = $this->buildEntity($form, $form_state);
+
+    $violations = $entity->validate();
+
+    if (!isset($form['uid']) || $form['uid']['#access'] == FALSE) {
+      $form_state->setTemporaryValue('name_violations', $violations->getByField('name'));
+    }
+    return parent::validateForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   protected function flagViolations(EntityConstraintViolationListInterface $violations, array $form, FormStateInterface $form_state) {
     // Manually flag violations of fields not handled by the form display.
     foreach ($violations->getByField('created') as $violation) {
       $form_state->setErrorByName('date', $violation->getMessage());
     }
-    foreach ($violations->getByField('name') as $violation) {
+    if (!$name_violations = $form_state->getTemporaryValue('name_violations')) {
+      $name_violations = $name_violations->getByField('name');
+    }
+    else {
+      $form_state->setTemporaryValue('name_violations', NULL);
+    }
+    foreach ($name_violations as $violation) {
       $form_state->setErrorByName('name', $violation->getMessage());
     }
     parent::flagViolations($violations, $form, $form_state);
diff --git a/core/modules/comment/src/Tests/CommentAnonymousTest.php b/core/modules/comment/src/Tests/CommentAnonymousTest.php
index 60e243b..9049731 100644
--- a/core/modules/comment/src/Tests/CommentAnonymousTest.php
+++ b/core/modules/comment/src/Tests/CommentAnonymousTest.php
@@ -44,6 +44,16 @@ function testAnonymous() {
     $anonymous_comment1 = $this->postComment($this->node, $this->randomMachineName(), $this->randomMachineName());
     $this->assertTrue($this->commentExists($anonymous_comment1), 'Anonymous comment without contact info found.');
 
+    // Ensure anonymous users cannot post in the name of registered users.
+    $edit = array(
+      'name' => $this->adminUser->getUsername(),
+      'comment_body[0][value]' => $this->randomMachineName(),
+    );
+    $this->drupalPostForm('comment/reply/node/' . $this->node->id() . '/comment', $edit, t('Save'));
+    $this->assertRaw(t('The name you used (%name) belongs to a registered user.', [
+      '%name' => $this->adminUser->getUsername(),
+    ]));
+
     // Allow contact info.
     $this->drupalLogin($this->adminUser);
     $this->setCommentAnonymous(COMMENT_ANONYMOUS_MAY_CONTACT);