This alert has been updated to clarify that versions 7.x of the Cisco ASA Software are not affected by the Cisco ASA Adaptive Security Appliance Software clientless SSL VPN Rot13-encoded cross-site scripting vulnerability.
Cisco ASA Adaptive Security Appliance Software versions prior to 8.0.4(34), 8.1.2(25), and 8.2.1(3) that have been configured to accept Clientless SSL VPN connections contain a cross-site scripting vulnerability that could allow an unauthenticated, remote attacker to execute arbitrary script or HTML code in a user's browser session. Versions 7.x are not affected.
The vulnerability is due to insufficient restrictions on Rot13-encoded URL parameters that are utilized by the SSL VPN feature of Cisco ASA Software when clients browse web pages by means of the VPN web portal. An attacker who could convince a user to visit a malicious page while logged into the secure portal could exploit this vulnerability to execute arbitrary script or HTML code in the security context of the affected site.
Cisco has confirmed this vulnerability and released updated software.
Cisco ASA Software versions prior to 8.0.4(34), 8.1.2(25), and 8.2.1(3) are affected when they are running on 5505, 5510, 5520, 5540, 5550, and 5580 devices.
Cisco ASA Software versions 7.x are not affected.
The vulnerability is due to a failure to properly protect the domain object model that is utilized by the Clientless SSL VPN against unauthorized modification. The vulnerability is likely to be exploited in instances where administrators allow users to enter arbitrary URLs that could be visited by means of the secure web portal. Systems that allow users to visit only the URLs that have been defined by administrators are less likely to be affected. In the latter case, an attacker would need to take control of a website that resides at one of the predefined URLs or perform URL spoofing or hijacking to conduct an attack.
Exploit code that demonstrates the cross-site scripting vulnerability is publicly available.
Cisco has released a Release Note Enclosure for Cisco bug ID CSCsy80705.
This vulnerability was reported to Cisco by Charles Henderson and David Byrne of Trustwave's SpiderLabs.
An unauthenticated, remote attacker could exploit this vulnerability to run arbitrary script or HTML code in the security context of the affected site. An exploit could allow the attacker to access sensitive browser based information such as authentication cookies or recently submitted data. An exploit could also allow an attacker to modify the appearance of the affected site.
The vulnerability is due to improper use of Rot13-encoded URL components that are utilized by the SSL VPN functionality of Cisco ASA Software. This processing of malicious URLs could allow for the execution of arbitrary script or HTML code.
An unauthenticated, remote attacker could exploit this vulnerability by convincing a user to view a malicious web page within the SSL VPN portal. A successful attack could allow the execution of arbitrary script or HTML code as well as modify the appearance of the affected web page.
Administrators are advised to apply the appropriate updates.
Administrators are advised to allow only trusted users to have network access.
Administrators are advised to configure the Clientless SSL VPN web portal to restrict users to administratively defined websites.
Administrators are advised to configure Web Access Control Lists (ACLs) to restrict users to internal or authorized resources.
Users are advised not to follow unsolicited links. Users should verify the authenticity of unexpected links prior to following them.
Users are advised not to visit websites or follow links that have suspicious characteristics or cannot be verified as safe.
Cisco customers with active contracts can obtain updates through the Software Center at the following link: Cisco. Cisco customers without contracts can obtain upgrades by contacting the Cisco Technical Assistance Center at 1-800-553-2447 or 1-408-526-7209 or via e-mail at email@example.com.
A special download page on the Software Center is available that contains fixed software releases at the following link: ASA-PSIRT
Version 1, June 24, 2009, 12:11 PM: Cisco ASA Adaptive Security Appliance Software contains a cross-site scripting vulnerability that could allow an unauthenticated, remote attacker to execute arbitrary script or HTML code in a user's browser. Updates are available.
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