JetEngine <= 3.8.6.1 - Unauthenticated SQL Injection via Listing Grid 'filtered_query' Parameter

The JetEngine plugin for WordPress is vulnerable to SQL Injection via the `listing_load_more` AJAX action in all versions up to, and including, 3.8.6.1. This is due to the `filtered_query` parameter being excluded from the HMAC signature validation (allowing attacker-controlled input to bypass security checks) combined with the `prepare_where_clause()` method in the SQL Query Builder not sanitizing the `compare` operator before concatenating it into SQL statements. This makes it possible for unauthenticated attackers to append additional SQL queries into already existing queries that can be used to extract sensitive information from the database, provided the site has a JetEngine Listing Grid with Load More enabled that uses a SQL Query Builder query.