A reflected cross-site scripting (XSS) vulnerability exists in Rukovoditel CRM version 3.6.4 and earlier in the Zadarma telephony API endpoint (/api/tel/zadarma.php). The application directly reflects user-supplied input from the 'zd_echo' GET parameter into the HTTP response without proper sanitization, output encoding, or content-type restrictions. The vulnerable code is: if (isset($_GET['zd_echo'])) exit($_GET['zd_echo']); An unauthenticated attacker can exploit this issue by crafting a malicious URL containing JavaScript payloads. When a victim visits the link, the payload executes in the context of the application within the victim's browser, potentially leading to session hijacking, credential theft, phishing, or account takeover. The issue is fixed in version 3.7, which introduces proper input validation and output encoding to prevent script injection.