A stored Cross-site Scripting (XSS) vulnerability exists in the MGate 5121/5122/5123 Series firmware version v1.0 because of insufficient sanitization and encoding of user input in the "Login Message" functionality. An authenticated attacker with administrative access can exploit this vulnerability to inject malicious scripts that are continuously stored on the device. These scripts are executed when other users access the login page, potentially resulting in unauthorized actions or other impacts, depending on the user's privileges.