IMPress for IDX Broker < 2.6.2 - Authenticated Stored Cross-Site Scripting (XSS) via unprotected 'idx_update_recaptcha_key' AJAX

Description

The IMPress for IDX Broker plugin contains a captcha feature to prevent spam submissions. Since it uses Google’s ReCAPTCHA service, it requires an API key. Unfortunately, the AJAX action the plugin registered to update this API key did not use capability checks or nonce checks.

This made it possible for a logged-in attacker with minimal permissions, such as a subscriber, to send a request to wp-admin/admin-ajax.php with the action parameter set to idx_update_recaptcha_key and the idx_recaptcha_site_key parameter set to a malicious JavaScript, which could then be executed in an administrator’s browser the next time they visited the plugin’s settings panel.