Nigel - added new references to the rule and bumped up revision number.
Rule:

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Sid:
1867

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Summary:
This event is generated when a remote user attempts to query the X Display Manager Control Protocol (XDMCP).

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Impact:
Reconnaissance.  An attacker may obtain a list of usernames on the remote host.

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Detailed Information:
The KDE Display Manager (KDM) provides a network protocol XDMCP to supply a graphical login screen.  It is possible to use this protocol to list the users on the remote host running XDMCP.  This provides reconnaissance and may be a precursor of attempting a brute force password attack of the revealed usernames.

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Affected Systems:
Any host running XDMCP.

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Attack Scenarios:
An attacker may obtain a list of current usernames on the remote host as a precursor of attempting a brute force attack to guess passwords of those users.

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Ease of Attack:
Simple.

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False Positives:
None.

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False Negatives:
None Known.

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Corrective Action:
Block inbound XDMCP traffic.

Disable XDMCP as a listening service on the remote host unless it is required.

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Contributors:
Original rule written by Max Vision <vision@whitehats.com>
Sourcefire Research Team
Judy Novak <judy.novak@sourcefire.com>

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Additional References:

Arachnids:
http://www.whitehats.com/info/IDS476

Nessus:
http://cgi.nessus.org/plugins/dump.php3?id=10891

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