The Page and Post Clone plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 6.0 via the 'content_clone' function due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with Author-level access and above, to clone and read private posts.
https://plugins.trac.wordpress.org/changeset/3108149/
https://plugins.trac.wordpress.org/browser/page-or-post-clone/trunk/page-or-post-clone.php#L19