File: 100000162.txt

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Rule: 

--
Sid: 
100000162

-- 
Summary: 
This event is generated when an abnormally larger number of 401 Unauthorized 
messages are returned by an SIP-enabled host.

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Impact:
This can be an indication of either a brute force authentication attack or a 
denial of service in progress.

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Detailed Information:
When a user attempts to send a REGISTER message with invalid credentials, a SIP 
server returns a 401 Unauthorized message. A high volume of these may indicate 
that an authentication attack, likely brute-force style, or a denial of service 
is in progress.

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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 attempt a brute-force authentication attack 
or a denial of service.

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Ease of Attack:
Simple, as it is easy to write a script to cycle through all possible 
authentication values or to simply flood a system with unauthorized data.

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False Positives:
None known.

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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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