[Snort-sigs] SID 625

Warchild warchild at ...288...
Fri Feb 15 18:30:02 EST 2002


Rule:
--
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN
XMAS";flags:SRAFPU; reference:arachnids,144;
classtype:attempted-recon; sid:625; rev:1;) 

Sid:
--
625

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

--
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.

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

--
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."
__
Ease of Attack:
Trivial.  Many of the popular portscanners/vulnerability testers, most
notably nmap, allow anyone to inititiate an XMAS scan.

--
False Positives:
This combination of flags should never be seen in "normal" activity,
so the chances of false positives are slim to none.

--
False Negatives:
None.

--
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.

--
Contributors:
Jon Hart <jhart at ...289...>
Warchild <warchild at ...288...>

-- 
Additional References:





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