Rule:

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Sid:
1892

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Summary:
This event is generated when SNMP communications contain a NULL value 
the authentication string.

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Impact:
Medium to Serious. Depending on if the community string was for 
read-only, read-create or read-write an attacker could gain a varying 
level of access to a system.

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Detailed Information:
An SNMP community string is the authentication process that a host 
running SNMP uses to grant access.

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Affected Systems:
Numerous. Routers, switches, servers, NAS systems, many others.

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Attack Scenarios:
An attacker can launch a scan of all network attached devices looking 
for port 161 (UDP) and then attempt to gain access using SNMP.

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Ease of Attack:
Simple. There are many free SNMP "tree walking" programs, an example of 
such is getIF.

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False Positives:
None Known

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False Negatives:
None Known

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Corrective Action:
Make sure that all devices that have SNMP turned on have complex 
passwords assigned.

Disable unneeded WRITE / CREATE community strings.

Since SNMP traffic is not encrypted, use a packet filtering firewall to 
restrict SNMP communications to the protected network.

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Contributors:
Sourcefire Research Team
Brian Caswell <bmc@sourcefire.com>
Nigel Houghton <nigel.houghton@sourcefire.com>
Snort documentation contributed by  Mike Rivett ebiz@rivett.org

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Additional References:

GetIF:
http://www.wtcs.org/snmp4tpc/getif.htm

CVE:
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-1999-0517

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