[Snort-sigs] SID 621

Warchild warchild at ...288...
Mon Mar 18 14:21:19 EST 2002

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN FIN"; flags:
F; reference:arachnids,27; classtype:attempted-recon; sid:621; rev:1;) 


A tcp packet with only it's FIN flag set was detected.

Information regarding firewall rulesets, open/closed ports, ACLs, and
possibly even OS type may be disclosed.  This technique can also be
used to bypass certain firewalls or traffic filtering/shaping devices.

Detailed Information:
A tcp packet with only it's FIN flag set was detected.  Most Windows
machines will respond with an ACK-RST regardless of whether or not the
port is open.  Most *nix systems will respond with an ACK-RST if the
port is closed and will not respond at all if the port is open.
Actual responses may vary.

Attack Scenarios:
As part of information gathering leading up to another (more directed)
attack, an attacker may attempt to figure out what ports are
open/closed on a remote machine.

Ease of Attack:
Intermediate.  To initiate an attack of this type, an attacker either
needs a tool that can send packets with only the FIN flag set or
the ability to craft their own packets.  The former is easy, the later
requires a more advanced skillset.

False Positives:

False Negatives:

Corrective Action:
Determine if this particular port would have responded as being open
or closed.  If open, watch for more attacks on this particular service
or from the remote machine that sent the packet.  If closed, simply
watch for more traffic from this host.  Consider filtering this type
of traffic at the ingress points of your network.

Warchild <warchild at ...288...>

Additional References:

More information about the Snort-sigs mailing list