[Full-disclosure] Cisco NAC Appliance Agent Installation Bypass Vulnerability

[Andreas Gal]

Description:
Cisco NAC Appliance (formerly Cisco Clean Access) is an easily deployed Network 
Admission Control (NAC) product that uses the network infrastructure to enforce 
security policy compliance on all devices seeking to access network computing 
resources. With NAC Appliance, network administrators can authenticate, 
authorize, evaluate, and remediate wired, wireless, and remote users and their 
machines prior to network access. It identifies whether networked devices such 
as laptops, IP phones, or game consoles are compliant with your network's 
security policies and repairs any vulnerabilities before permitting access to 
the network.

Vendor site:
http://www.cisco.com/en/US/products/ps6128/

Affected versions:
All current (<= 3.6.4.1 at the time of the release)

Discovery Date:
2006-08-15

Report Date:
2006-08-20 (vendor), 2006-08-25 (public)

Severity:
Medium

Remote:
Yes

Related previous reports:
http://www.securityfocus.com/archive/1/408603/30/0/threaded

Discovered by:
Andreas Gal (http://www.andreasgal.com/)
Joachim Feise (http://www.feise.com/)

Vulnerability:
Previous versions of the software allowed users to bypass the "mandatory" 
installation of the Clean Access Agent by changing the browser user-agent 
string. With version 3.6.0, Cisco added additional detection mechanisms such as 
TCP fingerprinting and JavaScript OS detection. By changing the default 
parameters of the Windows TCP/IP stack and using a custom HTTPS client (instead 
of a browser) the user can still connect to the network without running any 
host-based checks. Authentication and remote checks are not affected.

Proof-of-concept implementation:
http://kevin.sf.net/howto.html
http://kevin.sf.net/download/kevin.exe
http://kevin.sf.net/download/kevin.conf
http://kevin.cvs.sourceforge.net/kevin/

Acknowledgements:
The registry settings to masquerade the Windows TCP/IP stack were derived from 
sec_cloak written by Craig Heffner.