Rule:
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Sid:
521

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Summary:
This event is generated when an overly large UDP packet is observed. 

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Impact:
Possible denial of service.  UDP packet payloads are typically smaller than 4000 bytes.  One possible explanation of a payload of greater than 4000 bytes is an attempted denial of service.

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Detailed Information:
UDP payloads are typically smaller than 4000 bytes since the UDP protocol is intended to be used for the transmission of smaller payloads.  When a large payload is observed, it may be a sign or anomalous activity, perhaps an attempted denial of service against the remote host. 

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Affected Systems:
Any system that listens for a UDP service. 

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Attack Scenarios:
An attacker may craft large UDP payloads in an attempt to cause a denial of service against a remote host.

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Ease of Attack:
Simple.

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False Positives:
There may be UDP services offered that naturally support large payload sizes.

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False Negatives:
None Known.

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Corrective Action:
Allow only known UDP protocols inbound.

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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/IDS521

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