Contest Gallery <= 28.1.6 - Unauthenticated SQL Injection

The Contest Gallery plugin for WordPress is vulnerable to SQL Injection via the 'form_input' parameter in versions up to, and including, 28.1.6. This is due to insufficient escaping on the user supplied parameter and lack of sufficient preparation on the existing SQL query inside the unauthenticated 'post_cg_gallery_form_upload' AJAX action (specifically the 'cb' branch of the included users-upload-check.php, where $f_input_id is concatenated unquoted into 'SELECT Field_Content FROM ... WHERE id = $f_input_id'). The endpoint is gated only by a public frontend nonce ('cg1l_action' / 'cg_nonce') that is exposed in the page source of any public gallery page. 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.