Ninja Forms < 3.15.6 - Unauthenticated Stored XSS via 'extra' Parameter

Description

The plugin is vulnerable to Stored Cross-Site Scripting via the 'formData.extra' parameter 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 nf_ajax_submit endpoint issues a valid nonce to any requester on failure, effectively neutralizing the nonce check and making exploitation trivially achievable in a single unauthenticated request.