MW WP Form <= 5.1.7 - Unauthenticated Stored Cross-Site Scripting via 'post_id' Parameter (via stored form-submitted post meta)

The MW WP Form plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'post_id' parameter in all versions up to, and including, 5.1.7 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 CSRF gate protecting form submission (MW_WP_Form_Csrf) is bypassable by any unauthenticated visitor who first loads the public form page to obtain a valid double-submit cookie, leaving no effective barrier to storing malicious payloads.