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TACACS+ Authentication Bypass in Cisco Anomaly Detection and Mitigation
Products
------------------------------------------------------------------------
SUMMARY
<http://www.rfc-editor.org/cgi-bin/rfcdoctype.pl?loc=RFC&letsgo=1492&type=ftp&file_format=txt>
An Access Control Protocol is a new implementation of TACACS made by CISCO..
"
<http://www.cisco.com/en/US/products/ps5888/prod_bulletin0900aecd800fd124.html>
Cisco traffic anomaly detection and mitigation solutions deliver the industry's
most complete and powerful family of solutions for detecting and defeating
complex, sophisticated DDoS attacks."
A vulnerability in Cisco Anomaly Detection and Mitigation appliances and
service modules allows unauthorized users to gain access to the devices
and/or escalate their privileges if Terminal Access Controller Access
Control System Plus (TACACS+) is inadequately configured.
DETAILS
Vulnerable Systems:
* Cisco Anomaly Detection and Mitigation version 5.0(1)
* Cisco Anomaly Detection and Mitigation version 5.0(3)
The Cisco Guard and Cisco Traffic Anomaly Detector appliances and the
Anomaly Guard Module and Traffic Anomaly Detector Module for the Cisco
Catalyst 6500 switches/Cisco 7600 routers are Distributed Denial of
Service (DDoS) attack mitigation devices that detect the presence of a
potential DDoS attack and divert attack traffic destined for the network
being monitored without affecting the flow of legitimate traffic.
The Cisco Guard and the Cisco Anomaly Traffic Detector appliances can be
managed via a virtual terminal (standard keyboard and monitor attached
directly to the appliance), a local serial console, remote Secure Shell
(SSH) connections, and/or remote secure web sessions (HTTPS). The Anomaly
Guard Module and Traffic Anomaly Detector Module for the Cisco Catalyst
6500 switches/Cisco 7600 routers can be managed by logging into the module
from the switch (using the session command) as well as remotely via SSH
and/or secure web sessions.
TACACS+ is an authentication protocol that provides a way to centrally
validate users attempting to gain access to servers, workstations,
routers, switches, access servers, and other network devices.
Users accessing the Cisco Guard and the Cisco Anomaly Traffic Detector
devices can be authenticated against a local user database that is stored
in the device's configuration, or against an external TACACS+ server. A
complete configuration to authenticate users against an external TACACS+
server contains the following commands:
aaa authentication login tacacs+ local
aaa authentication enable tacacs+ local
tacacs-server host <IP address of TACACS+ server>
The aaa authentication login tacacs+ command configures TACACS+
authentication for users logging into the device via SSH or via the web
interface. The aaa authentication enable tacacs+ command configures
TACACS+ authentication for the enable command. The tacacs-server host
command specifies the TACACS+ server.
If the Cisco Guard and the Cisco Anomaly Traffic Detector devices are
configured to use an external TACACS+ server to authenticate users logging
into the device, but the actual TACACS+ server is not specified with
tacacs-server host command, then authentication will be bypassed.
Privileges that will be granted to the user that bypasses authentication
depend on type of account used to log in, and whether the account exists
on the device, as follows:
* Non-existent account used: user can only execute show commands.
* Existent local account used: user gets the same privileges that are
normally granted to that account.
* Existent Linux account used: user gets access to the underlying Linux
shell.
In addition, a user can bypass authentication of the enable command if
enable authentication is performed against a TACACS+ server (via the
command aaa authentication enable tacacs+) and the actual TACACS+ server
is not specified (via the tacacs-server host command.)
It is important to note that a device is vulnerable only if the
tacacs-server host command is missing. If this command is present the
device is not vulnerable, even if the IP address of the server is not
correct, and even if the TACACS+ server happens to be unreachable.
Successful exploitation of the vulnerability presented in this document
results in an authentication bypass, and may allow users to elevate the
privileges they have been given, allowing full control of the device.
Privilege elevation can potentially be used to sniff traffic, launch
Denial-of-Service (DoS) attacks, and to perform network reconnaissance by
inspection of the configuration policies.
Workarounds:
This vulnerability can be completely mitigated if the configuration of
TACACS+ authentication is completed by specifying the TACACS+ server via
the command tacacs-server host <IP address of TACACS+ server>.
As a security best practice, it is recommended that customers make use of
the access control feature that restricts connectivity to the SSH and
web-based management services to certain IP networks configured by the
administrator. This can be accomplished through the permit wbm and permit
ssh commands, which are documented in the following section of the
Configuration Guide:
<http://cisco.com/en/US/products/ps5888/products_configuration_guide_chapter09186a00804c0a6b.html#wp1162442>
http://cisco.com/en/US/products/ps5888/products_configuration_guide_chapter09186a00804c0a6b.html#wp1162442
Having these access control mechanisms in place may help mitigate the
vulnerability in the sense that only users coming from trusted networks
will be able to log in.
Vendor Status:
Cisco has made free software available to address this vulnerability for
affected customers. Prior to deploying software, customers should consult
their maintenance provider or check the software for feature set
compatibility and known issues specific to their environment.
Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise using
such software upgrades, customers agree to be bound by the terms of
Cisco's software license terms found at
<http://www.cisco.com/public/sw-license-agreement.html>
http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set
forth at Cisco.com Downloads at
<http://www.cisco.com/public/sw-center/sw-usingswc.shtml>
http://www.cisco.com/public/sw-center/sw-usingswc.shtml.
Do not contact either "psirt at cisco.com" or "security-alert at
cisco.com" for software upgrades.
ADDITIONAL INFORMATION
The information has been provided by <mailto:psirt@cisco.com> Cisco
Systems Security .
The original article can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20060215-guard.shtml>
http://www.cisco.com/warp/public/707/cisco-sa-20060215-guard.shtml
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