Rule:  
 
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Sid:

626

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Summary:
This event is generated when the Cybercop vulnerability scanner is used 
against a host.

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Impact:
Cybercop can be used to identify vulnerabilities on host systems.

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Detailed Information:
This particular packet is a part of Cybercop's OS identification.  
Specially crafted packets are able to elicit different responses from 
different operating systems.  This packet is likely to be part of a full
Cybercop scan rather than an isolated event. Having PUSH, ACK and 
reserve bits 1 and 2 set at the same time is unusual.  While this rule 
performs content as well as header checking to avoid false positives, 
this flag combination in the TCP header is possible is possible in a 
legitimate situation because of the addition of Explicit Congestion 
Notification (ECN).

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Affected Systems:
All

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Attack Scenarios:
Cybercop can be used by attackers to determine vulnerabilities present 
on a host or network of hosts that could be used as attack vectors.

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Ease of Attack:
Simple

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False Positives:
This tool can be used legitimately by a system and network 
administrators.

False positives from ECN enabled systems are possible.

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False Negatives:
None known.

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Corrective Action:
TCP packets with PUSH, ACK and reserved bits 1 and 2 set at the same 
time are unusual but possible with Explicit Congestion Notification 
(ECN).  It is advisable to block TCP packets with these flags set that 
do not have the ECT bit (TOS bit 6) set in the IP header.

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Contributors:
Original Rule Writer Unknown
Sourcefire Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>
Snort documentation contributed by Steven Alexander<alexander.s@mccd.edu>

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Additional References:

Arachnids:
http://www.whitehats.com/info/IDS149

Security Focus:
http://www.securityfocus.com/infocus/1205

RFC:
http://www.ietf.org/rfc/rfc2481.txt?number=2481

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