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

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Sid:
516

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Summary:
This event is generated when an attempt is made by Simple Network Management Protocol (SNMP) to enumerate Server Message Block (SMB) users on the host.

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Impact:
Reconnaissance.  An attacker may obtain SMB usernames of the remote host. 

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Detailed Information:
Server Message Block is a network file sharing protocol used between Windows hosts and Unix and between Windows hosts that communicate via Samba.  SNMP can be used to query a remote host that listens for SNMP requests and supports SMB, to list the SMB usernames.  This provides reconnaissance of valid usernames and may be followed by a brute force attack to guess passwords.

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Affected Systems:
Hosts that run SMB and listen for SNMP requests.

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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:
A Nessus script exists to list current SMB users.

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False Positives:
None.

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False Negatives:
None Known.

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Corrective Action:
Block inbound SNMP traffic.

Disable SNMP 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/IDS333

Nessus:
http://cgi.nessus.org/plugins/dump.php3?id=10546

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