Rule: 

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Sid: 
100000159

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Summary: 
This event is generated when an abnormally larger number of SIP REGISTER 
messages is received in a short time frame.

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Impact:
This can be an indication of a denial of service attack in progress, or simply 
a poorly configured or implemented user agent.

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Detailed Information:
This rule is used to detect overly large numbers of SIP REGISTER messages 
coming into hosts on an internal network, which may indicate a denial of 
service attack in progress. Since this traffic could also be the the result of 
a poorly configured user agent, or simply a very busy SIP proxy, careful 
analysis of both the hosts receiving and sending this traffic is required 
before determining that an attack has actually taken place.

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Affected Systems:
Any which implement the SIP protocol.

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Attack Scenarios:
An attacker could use a script to flood a system with REGISTER messages, 
causing a denial of service.

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Ease of Attack:
Simple, as SIP is a public, well-documented protocol.

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False Positives:
Known SIP proxies may receive a high volume of legitimate REGISTER requests, 
and NAT devices may appear to be sending a larger number of REGISTER requests 
than a regular host. It is recommended that users whitelist known SIP proxies 
and NAT devices.

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False Negatives:
None known.

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Corrective Action:
Use a firewall or other access-restriction device to block unwanted messages at 
your network's border.

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Contributors:
Jiri Markl <jiri.markl@nextsoft.cz>
Sourcefire Research Team

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Additional References
Other:

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