WordPress Gift Vouchers plugin before 4.1.8 contains a blind SQL injection vulnerability via the template_id parameter in a wp-admin/admin-ajax.php wpgv_doajax_front_template request. An attacker can possibly obtain sensitive information, modify data, and/or execute unauthorized administrative operations in the context of the affected site.
View the template here CVE-2018-16159.yaml
References:
https://www.exploit-db.com/exploits/45255/