[Snort-sigs] SID 625

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

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


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

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

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

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:

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.

Jon Hart <jhart at ...289...>
Warchild <warchild at ...288...>

Additional References:

More information about the Snort-sigs mailing list