Password Protected <= 2.8.4 - Authenticated (Subscriber+) Stored Cross-Site Scripting via Freemius purchase_completed Handler

The Password Protected plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the bundled Freemius SDK's purchase-completion handler in all versions up to, and including, 2.8.4 due to insufficient input sanitization and output escaping. Once the site's Freemius opt-in is complete (the normal state after an administrator clicks "Allow & Continue"), the handler processes the 'purchased_plan' request parameter on any wp-admin request from an authenticated user without any capability, nonce, or purchase validation, and stores it in a persistent, site-wide administrative notice. This makes it possible for authenticated attackers, with subscriber-level access and above, to inject arbitrary web scripts that execute automatically when an administrator views a standard admin page such as their Profile page.