The Payment forms, Buy now buttons, and Invoicing System | GetPaid plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'mmdwqzpoxadd' parameter in all versions up to, and including, 2.8.61 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The required nonce (getpaid_form_nonce) is publicly emitted to all visitors via wp_localize_script as WPInv.formNonce, meaning no authentication or prior nonce leak is needed; the stored payload fires when an administrator views the affected invoice's Payment Form Details metabox.