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Cisco PIX / CS ACS Downloadable RADIUS ACLs
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SUMMARY
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 (a.k.a. cut-through proxy) and VPN 3000
concentrators.
DETAILS
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:
1) User goes to Internet (for example) through the PIX via HTTP(s). PIX
asks a username and a password. User enters them into the dialog window.
2) PIX sends Radius Access-Request to CS ACS to authenticate the user (the
user password is encrypted by Radius).
3) 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.
4) PIX again sends Radius Access-Request to authenticate the user
#ACSACL#-IP-uacl-43a97a9d.
5) Radius server authenticates the user and sends back the ACL body as
another cisco-av-pair VSA attribute (ip:inacl#1= ...).
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.
ADDITIONAL INFORMATION
The information has been provided by <mailto:ovt@redcenter.ru> Oleg
Tipisov.
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