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
