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Cisco Security Advisory: TACACS+ Authentication Bypass in Cisco Anomaly Detection and Mitigation Products
From: Cisco Systems Product Security Incident Response Team <psirt () cisco com>
Date: Wed, 15 Feb 2006 17:00:00 +0100

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Cisco Security Advisory: TACACS+ Authentication Bypass in Cisco
Anomaly Detection and Mitigation Products

Document ID: 69073

Advisory ID: cisco-SA-20060215-guard-auth

http://www.cisco.com/warp/public/707/cisco-sa-20060215-guard.shtml

Revision 1.0
============

Last Updated 2006 February 15 1600 UTC (GMT)

For Public Release 2006 February 15 1600 UTC (GMT)

- -----------------------------------------------------------------

Contents
========

    Summary
    Affected Products
    Details
    Impact
    Software Versions and Fixes
    Workarounds
    Obtaining Fixed Software
    Exploitation and Public Announcements
    Status of This Notice: FINAL
    Distribution
    Revision History
    Cisco Security Procedures

- -----------------------------------------------------------------

Summary
=======

A vulnerability in versions 5.0(1) and 5.0(3) of the software used in
Cisco Anomaly Detection and Mitigation appliances and service modules
may allow unauthorized users to get unauthorized access to the
devices and/or escalate their privileges if Terminal Access
Controller Access Control System Plus (TACACS+) is incompletely
configured.

TACACS+ authentication is disabled by default, and a device correctly
configured for TACACS+ authentication is not affected by this
vulnerability.

Cisco has made free software available to address this vulnerability
for affected customers. There are workarounds available to mitigate
the effects of the vulnerability.

This advisory is posted at 
http://www.cisco.com/warp/public/707/cisco-sa-20060215-guard.shtml .

Affected Products
=================

Vulnerable Products
+------------------

This vulnerability affects versions 5.0(1) and 5.0(3) of the software
for 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 if the devices are
incompletely configured to use TACACS+ authentication. Please note
that 5.0(2) was never released to cisco.com, which is the reason it
is not listed as an affected release.

Devices running an affected software version and configured for
TACACS+ authentication are vulnerable if the Authentication,
Authorization, and Accounting (AAA) command specifies TACACS+
authentication but the configuration lacks the tacacs-server host
command that specifies the TACACS+ server. In other words, if the
configuration includes either or both of the following commands:

    aaa authentication login tacacs+ local
    aaa authentication enable tacacs+ local


but not the following command:

    tacacs-server host <IP address of TACACS+ server>


the device is vulnerable.

Note:  The "local" authentication method specified after the
"tacacs+" authentication method in the aaa authentication commands
above is unrelated to the vulnerability. This authentication method
is shown because it is normally used as a fallback in case the
TACACS+ server is not available. Devices maybe vulnerable, with or
without a "local" authentication method, if the "tacacs+"
authentication method is used before the "local" method (if
specified) and the configuration lacks the tacacs-server host
command.

Products Confirmed Not Vulnerable
+--------------------------------

The Cisco Guard and Cisco Traffic Anomaly Detector are not affected
by this vulnerability if they are running the following software
versions:

  * Versions of the Cisco Guard and Cisco Traffic Anomaly Detector
    software prior to 5.0. This includes any 3.x and 4.x release.
  * Cisco Guard and Cisco Traffic Anomaly Detector software version
    5.1 and above.

A Cisco Guard or Cisco Traffic Anomaly Detector running version 5.0
(1) or 5.0(3) is not affected if the device is not configured to
authenticate users against a TACACS+ server, or if its TACACS+
configuration is complete, i.e. if the tacacs-server host command is
present in the configuration.

Note:  TACACS+ authentication is disabled by default. If no explicit
AAA configuration takes place the Cisco Guard and the Cisco Traffic
Anomaly Detector will authenticate users against the local database
(the "local" authentication method.)

No other Cisco products are currently known to be affected by this
vulnerability.

Details
=======

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.

This vulnerability is documented in Cisco bug ID CSCsd21455.

Impact
======

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.

Software Versions and Fixes
===========================

This vulnerability has been resolved in the 5.1 series of the Cisco
Guard and Cisco Traffic Anomaly Detector software. The first release
in the 5.1 series is 5.1(4).

Software for the Cisco Guard appliance is available for download at
http://www.cisco.com/pcgi-bin/tablebuild.pl/cisco-ga-crypto.

Software for the Cisco Traffic Anomaly Detector appliance is
available for download at 
http://www.cisco.com/pcgi-bin/tablebuild.pl/cisco-ad-crypto.

Software for the Cisco Anomaly Guard Module for the Cisco Catalyst
6500 switches/Cisco 7600 routers is available for download at 
http://www.cisco.com/pcgi-bin/tablebuild.pl/cisco-agm-crypto.

Software for the Cisco Anomaly Traffic Detector Module for the Cisco
Catalyst 6500 switches/Cisco 7600 routers is available for download
at http://www.cisco.com/pcgi-bin/tablebuild.pl/cisco-adm-crypto.

When considering software upgrades, also consult 
http://www.cisco.com/go/psirt and any subsequent advisories to 
determine exposure and a complete upgrade solution.

In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current
hardware and software configurations will continue to be supported
properly by the new release. If the information is not clear, contact
the Cisco Technical Assistance Center ("TAC") or your contracted
maintenance provider for assistance.

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

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.

Obtaining Fixed Software
========================

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, or as otherwise 
set forth at Cisco.com Downloads at 
http://www.cisco.com/public/sw-center/sw-usingswc.shtml.

Do not contact either "psirt () cisco com" or "security-alert () cisco com"
for software upgrades.

Customers with Service Contracts
+-------------------------------

Customers with contracts should obtain upgraded software through
their regular update channels. For most customers, this means that
upgrades should be obtained through the Software Center on Cisco's
worldwide website at http://www.cisco.com.

Customers using Third-party Support Organizations
+------------------------------------------------

Customers whose Cisco products are provided or maintained through
prior or existing agreement with third-party support organizations
such as Cisco Partners, authorized resellers, or service providers
should contact that support organization for guidance and assistance
with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic
behavior, and organizational mission. Due to the variety of affected
products and releases, customers should consult with their service
provider or support organization to ensure any applied workaround or
fix is the most appropriate for use in the intended network before it
is deployed.

Customers without Service Contracts
+----------------------------------

Customers who purchase direct from Cisco but who do not hold a Cisco
service contract and customers who purchase through third-party
vendors but are unsuccessful at obtaining fixed software through
their point of sale should get their upgrades by contacting the Cisco
Technical Assistance Center (TAC). TAC contacts are as follows.

  * +1 800 553 2447 (toll free from within North America)
  * +1 408 526 7209 (toll call from anywhere in the world)
  * e-mail: tac () cisco com

Have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free
upgrades for non-contract customers must be requested through the
TAC.

Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml
for additional TAC contact information, including special localized
telephone numbers and instructions and e-mail addresses for use in
various languages.

Exploitation and Public Announcements
=====================================

The Cisco PSIRT is not aware of any public announcements or malicious
use of the vulnerability described in this advisory.

Cisco would like to thank Gerrit Wenig from Verizon Business for
bringing this issue to our attention.

Status of This Notice: FINAL
============================

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY
KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE
INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS
AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS
DOCUMENT AT ANY TIME.

A stand-alone copy or Paraphrase of the text of this document that
omits the distribution URL in the following section is an
uncontrolled copy, and may lack important information or contain
factual errors.

Distribution
============

This advisory is posted on Cisco's worldwide website at 
http://www.cisco.com/warp/public/707/cisco-sa-20060215-guard.shtml.

In addition to worldwide web posting, a text version of this notice
is clear-signed with the Cisco PSIRT PGP key and is posted to the
following e-mail and Usenet news recipients.

  * cust-security-announce () cisco com
  * first-teams () first org
  * bugtraq () securityfocus com
  * vulnwatch () vulnwatch org
  * cisco () spot colorado edu
  * cisco-nsp () puck nether net
  * full-disclosure () lists grok org uk
  * comp.dcom.sys.cisco () newsgate cisco com

Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on
mailing lists or newsgroups. Users concerned about this problem are
encouraged to check the above URL for any updates.

Revision History
================

+---------------------------------------+
| Revision | 2006-February-15 | Initial |
| 1.0      |                  | Release |
+---------------------------------------+

Cisco Security Procedures
=========================

Complete information on reporting security vulnerabilities in Cisco
products, obtaining assistance with security incidents, and
registering to receive security information from Cisco, is available
on Cisco's worldwide website at 
http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html.
This includes instructions for press inquiries regarding Cisco security 
notices. All Cisco security advisories are available at 
http://www.cisco.com/go/psirt.

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