Cisco PIX / CS ACS: Downloadable RADIUS ACLs vulnerability

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Hi!

The following is the description of the vulnerability in the Cisco implementation of downloadable ACLs, which are used 
by the Cisco PIX firewall authentication proxy (aka cut-through proxy) and VPN 3000 concentrators.

When an administrator creates an ACL on the Cisco Secure Access Control Server (CS ACS Radius server) it is assigned 
the internal name #ACSACL#-IP-uacl-<random>. For example, the name may be the following: #ACSACL#-IP-uacl-43a97a9d. The 
<random> is changed by CS ACS every time the ACL is modified by the administrator. At the same time the internal hidden 
user with the name #ACSACL#-IP-uacl-43a97a9d and the password #ACSACL#-IP-uacl-43a97a9d (!) is created by CS ACS. This 
user is not seen in the CS ACS GUI.

The protocol used by the PIX to download the ACL works as follows: 0) User goes to Internet (for example) thru the PIX 
via HTTP(s). PIX asks a username and a password. User enters them into the dialog window. 1) PIX sends Radius 
Access-Request to CS ACS to authenticate the user (the user password is encrypted by Radius). 2) Radius server 
authenticates the user and sends back the cisco-av-pair Vendor-specific attribute (VSA) with the value 
ACS:CiscoSecure-Defined-ACL=#ACSACL#-IP-uacl-43a97a9d. 3) PIX again sends Radius Access-Request to authenticate the 
user #ACSACL#-IP-uacl-43a97a9d. 4) Radius server authenticates the user and sends back the ACL body as another 
cisco-av-pair VSA attribute (ip:inacl#1= ...).

Vulnerability:

This basically means that everybody with a sniffer can see the username #ACSACL#-IP-uacl-43a97a9d which is sent over 
the network in clear by the Radius protocol from the CS ACS server to the PIX. The password of this user is the same as 
the username. If some network device is configured to use the very same CS ACS server for login authentication then the 
sniffed username can be used to login to this network device.

Setting Radius IETF attribute Service-type to "Outbound" to prevent using this username for logins may not help: 1) 
it's impossible to set this attribute for the user #ACSACL#-IP-uacl-43a97a9d, because the user is not seen in the CS 
ACS Web interface 2) it's not always possible to set it for the "default" group (the user #ACSACL#-IP-uacl-43a97a9d 
always belongs to the "default" CS ACS group), because this group may be used for something else 3) some network 
devices (most notably the PIX firewall) ignore the Service-Type attribute (PIX firewall 6.x code does not support login 
authorization at all (!)). Cisco routers ignore this attribute if authorization is not configured (only authentication 
is configured).

Generally speaking the Radius protocol is not appropriate for doing such things as downloading ACLs or other attributes 
on behalf of the user on an "as-needed" basis, as it doesn't separate the authentication and authorization. Usually 
this leads to creation of a fake user with the password "cisco" or "<username>". Unfortunately this practice is common 
on Cisco devices.

Thx,
Oleg Tipisov,
Moscow