Rule:

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Sid:
2041

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Summary:
The Extended Terminal Access Controller Access Control System (XTACACS) 
is an authentication and authorization protocol derived from  CISCO 
TACACS. It is used in TCP/IP networks where network servers authenticate
clients from a master server.

This event is generated when a failed login using XTACACS is observed.

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Impact:
This may be an intelligence gathering activity or an attempt to access 
resources controlled by the XTACACS server.

Multiple events from this rule may indicate the attempted enumeration of
a valid user account using brute force methodology.

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Detailed Information:
When a user logs in to a server that uses XTACACS the server then makes 
a request to a master server to determine the validity of the request. 
The master server then verifies the login attempt and returns data 
concerning that user which may include information regarding resources 
the user is allowed access to in the form of an access list.

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Affected Systems:
All servers using XTACACS for authentication control.

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Attack Scenarios:
Regular user login method.

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Ease of Attack:
Simple

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False Positives:
This rule may generate an event when a legitimate user supplies an
incorrect password or username when logging in to a device.

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False Negatives:
None Known

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Corrective Action:
XTACACS servers should only authenticate to known hosts and firewall 
rules should prevent access to XTACACS enabled servers from outside the 
local area network.

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Contributors:
Sourcefire Research Team
Brian Caswell <bmc@sourcefire.com>
Nigel Houghton <nigel.houghton@sourcefire.com>

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Additional References:

Network Information Library - Intel:
http://www.intel.com/support/si/library/bi0414.htm

The Internet Next Generation Project:
http://ing.ctit.utwente.nl/WU5/D5.1/Technology/xtacacs/

Xtacacs Home:
http://www.netplex-tech.com/software/xtacacsd/

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