WP Page Permalink Extension <= 1.5.4 - Missing Authorization to Authenticated (Subscriber+) Arbitrary Rewrite Rules Flush

The WP Page Permalink Extension plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 1.5.4. This is due to missing authorization checks on the `cwpp_trigger_flush_rewrite_rules` function hooked to `wp_ajax_cwpp_trigger_flush_rewrite_rules`. This makes it possible for authenticated attackers, with Subscriber-level access and above, to flush the site's rewrite rules via the `action` parameter.