FundEngine <= 1.8.1 - Missing Authorization to Authenticated (Subscriber+) Arbitrary Post Modification via 'campaign_post' Parameter

The FundEngine – Donation and Crowdfunding Platform plugin for WordPress is vulnerable to authorization bypass in all versions up to, and including, 1.8.1. This is due to the plugin not properly verifying that a user is authorized to perform an action. This makes it possible for authenticated attackers, with subscriber-level access and above, to modify arbitrary posts and pages — overwriting title and content, and seizing ownership by supplying an attacker-controlled post_author integer that bypasses wp_kses_post sanitization. The wp_rest nonce required by the handler is trivially obtainable by any logged-in user via /wp-admin/admin-ajax.php?action=rest-nonce and therefore does not constitute an authorization barrier.