Cross-Site Scripting (XSS) vulnerability was discovered in the GSVoIP web panel version 2.0.90. The `msg` parameter in the `/painel/gateways.php/error` endpoint does not properly sanitize user-supplied input, allowing attackers to inject arbitrary JavaScript into the HTML response. A remote attacker can exploit this vulnerability by sending a crafted URL to a victim, leading to unauthorized script execution, session hijacking, phishing, or other client-side attacks.