A vulnerability in the web-based interface of the Cisco Registered Envelope Service could allow an authenticated, remote attacker to conduct a cross-site scripting (XSS) attack against another user of the service. The vulnerability is due to insufficient validation of user-supplied input by the web-based interface of the affected software. An attacker could exploit this vulnerability by sending an email with a malicious payload to another user. A successful exploit could allow the attacker to execute arbitrary script code in the context of the affected interface or access sensitive, browser-based information. This vulnerability affects software versions 5.3.4.x.