Rule:
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Sid:
625

Summary:
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A TCP packet with all of the (unreserved) control bits set was
detected as being destined for your machine. 

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Impact:
System recon.  Different operating-systems will respond in different
ways depending on their particular stack implementation.  This allows
attackers to determine things such as open/closed ports, ACLs, and the
like.

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Detailed Information:
The ACK, FIN, PSH, RST, SYN, and URG control bits were set in a TCP
packet.  

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Affected Systems:
 
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Attack Scenarios:
As part of a recon mission that may be an indicator to upcoming
attacks, an attacker may attempt to determine what ports are listening
on a given machine by sending a TCP packet with all of its control
bits "lit up", hence the name XMAS scan -- its "lit up like a
christmas tree."
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Ease of Attack:
Trivial.  Many of the popular portscanners/vulnerability testers, most
notably nmap, allow anyone to inititiate an XMAS scan.

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False Positives:
None Known

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False Negatives:
None Known

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Corrective Action:
Determine what information an attacker may have gleaned from this
attack.  Would your ports show as open or closed?  Consider
implementing a stateful firewall on the victim machine, or at ingress
points on your network.

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Contributors:
Original rule writer unknown
Original document author unkown
Sourcefire Vulnerability Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>
Jon Hart <warchild@spoofed.org>

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Additional References:
http://rr.sans.org/firewall/egress.php

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