A vulnerability in the Web Server Authentication Required screen of the Clientless Secure Sockets Layer (SSL) VPN portal of Cisco Adaptive Security Appliance (ASA) Software could allow an unauthenticated, remote attacker to conduct a cross-site scripting (XSS) attack against a user of that portal on an affected device. The vulnerability is due to insufficient validation of user-supplied input by the web-based interface of an affected device. An attacker could exploit this vulnerability by persuading a user of the interface to click a crafted link. A successful exploit could allow the attacker to execute arbitrary script code in the context of the portal or allow the attacker to access sensitive browser-based information. This vulnerability affects the following Cisco products if they are running a vulnerable release of Cisco ASA Software: 3000 Series Industrial Security Appliances, Adaptive Security Virtual Appliance (ASAv), ASA 5500 Series Adaptive Security Appliances, ASA 5500-X Series Next-Generation Firewalls, ASA Services Module for Cisco Catalyst 6500 Series Switches, ASA Services Module for Cisco 7600 Series Routers. Cisco Bug IDs: CSCvh20742.