Kentico CMS 9.0-12.0.49 - Persistent Cross Site Scripting

# Exploit Title: Kentico CMS 9.0-12.0.49 - Persistent Cross Site Scripting
# Exploit Author: Ataberk YAVUZER
# CVE: CVE-2019-19493
# Type: Webapps
# Vendor Homepage: https://www.kentico.com/
# Version: 9.0-12.0.49
# Date: 29-11-2019

#CVE Details: https://nvd.nist.gov/vuln/detail/CVE-2019-19493

Details

Persistent Cross Site Scripting vulnerability has been found on the
Admin/User Panel. Kentico before 12.0.50 allows file uploads in which the
Content-Type header is inconsistent with the file extension, leading to XSS.

# Steps to reproduce

   1. Log in to Kentico Admin Panel with your credentials.
   2. Browse to Profile Page.
   3. Click to "Browse" button on Avatar section.
   4. Select "avatar.svg" file which can be found on below.
   5. Intercept the request before clicking to save button.
   6. Change file name to "avatar.svg.png" and send the request. (MimeType
   needs to be "image/xml+svg")
   7. Kentico will generate an avatar link: "
   http://example.kentico.com/admin/CMSPages/GetAvatar.aspx?avatarguid=<generated_avatar_uid>"
   Send that link to another user.
   8. An alert with cookie values will pop up.


#Content of the avatar.svg:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<svg xmlns="http://www.w3.org/2000/svg" onload="alert(document.cookie)"/>