#3421381 added condition for valid submission data.

src/Plugin/rest/resource/WebformSubmitResource.php

@@ -138,6 +138,17 @@ class WebformSubmitResource extends ResourceBase {
       return $this->dispatchReturnEvent([], $errors, 'error', 400);
     }
 
+    // Check for a valid webform submission data.
+    $elements = $webform->getElementsDecoded();
+    if (array_diff_key($values['data'], $elements)) {
+      $errors = [
+        'error' => [
+          'message' => $this->t('Invalid submission data.'),
+        ],
+      ];
+      return $this->dispatchReturnEvent([], $errors, 'error', 400);
+    }
+
     //Check if webform allows drafts
     $allow_draft = $webform->getSetting('draft');
       if(isset($webform_data['draft']) && $allow_draft === 'none' && $webform_data['draft'] === TRUE){