A remote SQL injection vulnerability exists in the WordPress plugin stnl_iframe.php. An attacker can exploit this vulnerability to gain access to the database and extract sensitive information such as usernames, passwords, and emails. The vulnerability is due to insufficient sanitization of user-supplied input to the 'newsletter' parameter. An attacker can exploit this vulnerability by sending a specially crafted HTTP request containing malicious SQL statements to the vulnerable script. This will allow the attacker to gain access to the database and extract sensitive information.