Cisco Security Advisory-
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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20060215-guard.
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This section provides details on 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:
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Versions of the Cisco Guard and Cisco Traffic Anomaly Detector
software prior to 5.0. This includes any 3.x and 4.x release.
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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.
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Versions of the Cisco Guard and Cisco Traffic Anomaly Detector
software prior to 5.0. This includes any 3.x and 4.x release.
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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:
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Non-existent account used: user can only execute
show commands.
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Existent local account used: user gets the same privileges that are
normally granted to that account.
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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 ( registered customers only) .
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Non-existent account used: user can only execute
show commands.
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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:
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.
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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.
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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.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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