I just wanted to know what exactly is the FIN attack. I know about the FIN flag that is used to indicated the closing of a connection via TCP. But what exactly is FIN attack?

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    Have you done any research on your own? How did you hear about the "FIN attack"?
    – schroeder
    Commented Feb 12, 2015 at 1:02
  • Basically it was given in an assignment. HAve been doing some reading but have only come across FIN as a flag in the TCP header. But nothing about a FIN attack. Commented Feb 12, 2015 at 2:20
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    Then could you get some more details from your instructor about the term "FIN Attack"? There are scans and floods, but I'm not sure that I would describe either of those as "attacks"
    – schroeder
    Commented Feb 12, 2015 at 18:38
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    If you read the 2 questions to this answer, you will see that they are assuming that you mean a FIN scan. Scans are not attacks. Do you mean FIN scan, or are you still not getting the answers you need...?
    – schroeder
    Commented Feb 12, 2015 at 20:22
  • Yeah, i figured that much. Still haven't gonna much info on it but i figure its basically using the FIN packets to find a hole somewhere since it sometimes can be able to bypass firewalls. Commented Feb 13, 2015 at 16:22

3 Answers 3


It's an older attack originally intended to be a "sneaky, firewall bypass" that was dependent on a few factors that are now uncommon today: old Unix OSes, lack of stateful firewalls, lack of NIDS/NIPS, etc. It can still be useful when testing (i.e., as a fingerprinting technique not an attack per se) completely new or novel TCP/IP stacks (or just new to you or your environment), which is rare but can happen.

Here is a modern replacement, the TCP protocol scan:

nmap --reason -n -Pn --packet-trace -g 80 -sO -p 6 <target ip>

Which is almost exactly the same as the TCP ACK scan (which can be used to map hosts, open ports, firewall rulesets, etc with the caveat that some NIPS, IDS, and modern firewalls will detect -- with another situation-specific event where perhaps it will not notify incident responders or Security Operations Centers because they have more important things to look at these days):

nmap --reason -n -Pn --packet-trace -g 80 -sA -p 80 <target ip>

But the outputs are slightly different and you can see the other packet-level differences as well.

What you are looking for in order to develop a more advanced technique is to identify the subtleties in the RST packets and their window sizes. If you get non-zero window sizes, then you may want to switch to using the TCP Window scan instead of the TCP ACK scan. For more information, see http://nmap.org/book/man-port-scanning-techniques.html

Some other techniques are found in the NSE guide, such as the firewalk and firewall-bypass scripts. However, there are many other techniques including BNAT, fragroute, osstmm-afd, 0trace, lft, and potentially others that detect other inline, non-firewall devices such as WAFs, IDS, IPS, reverse proxies, gateways, and deception systems such as honeypots or active defenses. You will want to be aware of all of this and more if you are performing a network penetration test, but they come in handy for troubleshooting all sorts of network and security issues.

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    I appreciate your answer but i still don't get what a FIN attack is though. Oh, love Nmaps though :D Commented Feb 12, 2015 at 2:24
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    I think the short version is, you send a FIN that doesn't belong to a session and learn from the response you get. The rest of @atdre's answer is good enough I'd like to just see him add this detail.
    – gowenfawr
    Commented Feb 12, 2015 at 3:09
  • Yeah, sounds just about right.. Just trying to figure how to use the FIN scan in an attack manner. Commented Feb 13, 2015 at 16:26

FIN Attack(I assume you mean FIN Scan) is a type of TCP Port Scanning.

According to RFC 793: "Traffic to a closed port should always return RST". RFC 793 also states if a port is open and segment does not have flag SYN, RST or ACK set. The packet should be dropped. It could be an old datagram from an already closed session.

So what the FIN Attack does is to abuse this. If we send a FIN packet to a closed port we get a RST back. If we get no response we know that is either dropped by the firewall or the port is open. Also, the FIN Attack is more invisible than the SYN Scan(sending SYN to see the respons).

However, many system always return RST. And then it is not possible to know if the port is open or closed, for example Windows does this but not UNIX.

  • Hmmmmm, so its basically sent to get a response so the "attacker" and know what to do? Commented Feb 12, 2015 at 15:30
  • Yes, you probe the target machine for open ports. This could be performed by an admin or an attacker to find vulnerabilities. Commented Feb 13, 2015 at 8:16
  • Oh, yes.. I was thinking the same thing. But needed some confirmation. Commented Feb 13, 2015 at 16:28

FIN scans, as NULL, XMAS, or custom-flags scans --were and-- are used for bypassing firewall and sometimes evading IDS, I quote:

FIN Scan: The key advantage to these scan types is that they can sneak through certain non-stateful firewalls and packet filtering routers. Such firewalls try to prevent incoming TCP connections (while allowing outbound ones) Demonstrating the full , firewall-bypassing power of these scans requires a rather lame target firewall configuration. With a modern stateful firewall, a FIN scan should not produce any extra information.

SYN/FIN One interesting custom scan type is SYN/FIN. Sometimes a firewall administrator or device manufacturer will attempt to block incoming connections with a rule such as "drop any incoming packets with only the SYN Hag set". They limit it to only the SYN flag because they don't want to block the SYN/ACK packets which are returned as the second step of an outgoing connection. The problem with this approach is that most end systems will accept initial SYN packets which contain other (non-ACK) flags as well. For example, the Nmap OS fingerprinting system sends a SYN/FIN/URG/PSH packet to an open port. More than half of the fingerprints in the database respond with a SYN/ACK. Thus they allow port scanning with this packet and generally allow making a full TCP connection too. Some systems have even been known to respond with SYN/ACK to a SYN/RST packet! The TCP RFC is ambiguous as to which flags are acceptable in an initial SYN packet, though SYN/RST certainly seems bogus. Example 5.13 shows Ereet conducting a successful SYNIFIN scan of Google. He is apparently getting bored with scanme.nmap.org.

NMAP Network Discovery by Gordon "Fyodor" Lyon

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