Ha! I know how to fix. You can try http://(your domain)/wp-includes/js/tinymce/wp-tinymce.php?c=1&ver=4109-20150505
If it returns 403 Forbidden HTTP status code, you need to open the execution permission of wp-tinymce.php to make main editing box workable.
Thanks wapphotography & PHPMatters. I don’t have install any plugins and just one theme called Divi.
I guess the issue comes from some Visual Editor(TinyMCE) vulnerability is fixed from the information below.
https://codex.wordpress.org/Version_4.2.2
4.2.1 is okay. The error happens after updated to 4.2.2.
