Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 53448)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -2801,7 +2801,7 @@
  * @deprecated 4.3.0
  */
 function wp_ajax_wp_fullscreen_save_post() {
-	$post_id = isset( $_POST['post_ID'] ) ? (int) $_POST['post_ID'] : 0;
+	$post_id = filter_input(INPUT_POST, 'post_ID', FILTER_SANITIZE_STRING) !== null ? (int) filter_input(INPUT_POST, 'post_ID', FILTER_SANITIZE_STRING) : 0;
 
 	$post = null;