The plugin does not sanitise and escape some of its settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup).
1. Login as Admin.
2. Go to http://172.28.128.6/wordpress/wp-admin/options-general.php?page=wp-custom-author-url-settings
3. Paste the payload `" onfocus="alert(1)" autofocus="` in the Redirect URL field.
4. Save the settings.
5. The XSS will be triggered everytime you hover on the input field.