Tcp fin scan

The table below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.

The table below shows the views that this attack pattern belongs to and top level categories within that view. The table below specifies different individual consequences associated with the attack pattern.

The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses but not necessarily all may be present for the attack to be successful. Each related weakness is identified by a CWE identifier. Use of the Common Attack Pattern Enumeration and Classification dictionary and classification taxonomy, and the associated references from this website, are subject to the Terms of Use.

For more information, please email capec mitre. Attack Pattern ID: Presentation Filter:. The RFC expected behavior is that any TCP segment with an out-of-state Flag sent to an open port is discarded, whereas segments with out-of-state flags sent to closed ports should be handled with a RST in response. This behavior should allow the adversary to scan for closed ports by sending certain types of rule-breaking packets out of sync or disallowed by the TCB and detect closed ports via RST packets.

Such filters are configured to block access to ports usually by preventing SYN packets, thus stopping any attempt to 'build' a connection.

Many operating systems, however, do not implement RFC exactly and for this reason FIN scans do not work as expected against these devices.

tcp fin scan

Some operating systems, like Microsoft Windows, send a RST packet in response to any out-of-sync or malformed TCP segments received by a listening socket rather than dropping the packet via RFCthus preventing an attacker from distinguishing between open and closed ports.

FIN scans are limited by the range of platforms against which they work.

5 Basic Port Scanning Techniques

Additionally, because open ports are inferred via no responses being generated, one cannot distinguish an open port from a filtered port without further analysis. For instance, FIN scanning a system protected by a stateful firewall may indicate all ports being open. For these reasons, FIN scanning results must always be interpreted as part of a larger scanning strategy.

FIN scanning is still relatively stealthy as the packets tend to blend in with the background noise on a network link. FIN scans are detected via heuristic non-signature based algorithms, much in the same way as other scan types are detected.

Typical Severity. It is often seen as a singular piece of a fully executed attack.

Port scanner

A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern. On Unix and Linux, raw socket manipulations require root privileges. Resources Required. This can be achieved via the use of a network mapper or scanner, or via raw socket programming in a scripting language.

Packet injection tools are also useful for this purpose.

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Depending upon the method used it may be necessary to sniff the network in order to see the response. Scope Impact Likelihood Confidentiality. Related Weaknesses. Chapter 2: Scanning, pg.

what is "Fin Scan"? How harmful is it?

McGraw Hill.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Information Security Stack Exchange is a question and answer site for information security professionals. It only takes a minute to sign up. I just wanted to know what exactly is the FIN attack. But what exactly is FIN attack? It can still be useful when testing i.

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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 :.

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. Some other techniques are found in the NSE guidesuch as the firewalk and firewall-bypass scripts.

tcp fin scan

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. 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 get no response we know that is either dropped by the firewall or the port is open. 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.

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 fullfirewall-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. 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".

The problem with this approach is that most end systems will accept initial SYN packets which contain other non-ACK flags as well. Thus they allow port scanning with this packet and generally allow making a full TCP connection too. Example 5.Choose a Session. Data Security. Jeff Petters. Imagine a long hallway with doors on either side. There are a total ofdoors. This is what a cybercriminal might see when they look at one of your computers, except they can look through many different hallways and all the doors at the same time.

Are you watching all of the doors? Some of the doors, maybe? Are you using the same port scanning techniques the cybercriminals would use to see where you might be vulnerable to attacks? You should be. A port scanner is a simple computer program that checks all of those doors — which we will start calling ports — and responds with one of three possible responses: Open, Closed, or Filtered.

Anything above is available for use by services or applications. Cybercriminals use a port scanner to find potential weak points they could exploit, with malware or a Trojan on that system, or to use that computer to connect to other systems in your network.

Port scanning is quite simple: a port scanner sends a request to connect to a port on a computer and records the response.

tcp fin scan

Cybercriminals are looking for open ports that they can use as communication relays or infiltration vectors into your network. Any open port they can find is a possible access point for further infiltration into your network. The simplest port scans are ping scans. A ping scan is an automated blast of many ICMP echo requests to different targets to see who responds. Administrators usually disable ping either on the firewall or on the router.

However, ping is a good troubleshooting tool, and turning it off makes tracking down network problems a little more difficult.

This scan is fast because it never completes the full TCP 3 way-handshake. No responses indicate SYN is filtered on the network. This port scanning technique is basically the same as the TCP Half-Open scan, but instead of leaving the target hanging, the port scanner completes the TCP connection.

First, you have to send one more packet per scan, which increases the amount of noise you are making on the network. When you run a UDP port scan, you send either an empty packet or a packet that has a different payload per port, depending on your use case. The trick with a UDP scan is that you will only get a response if the port is closed, which means you might know that there is a computer there.

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You could be waiting a while to get a response that might never come. If you do get a reply, you know that there is a DNS server on that computer. A UDP scan can be useful to scout for active services that way, and the nmap port scanner is preconfigured to send requests for many standard services.

Transmission Control Protocol TCP is a nice orderly transaction protocol: TCP sends each packet in order, complete with error checking, verification, and a 3-way handshake to confirm each packet is successful.

Programs that use UDP just send the data — and if you miss a packet, you will never get it again. Sometimes a hacker whitehat or blackhat wants to run a port scan that is even quieter and less obvious than the other kinds of scans.

Thankfully, TCP includes some flags that allow you to do just that. When you send a port scan with a packet and the FIN flag, you are sending the packet and not expecting a response.

If you do get an RST you can assume that the port is closed. If you get nothing back that indicates the port is open. The biggest advantage of using these flags is that they can slip past the firewall, which makes the results more reliable.A TCP SYN scan which we have covered earlier leaves a lot of fingerprints on the target host, thus revealing the identity of the scanning host.

So how does a penetration tester work around this? The Nmap FIN scan comes in handy in such circumstances. Since there is no earlier communication between the scanning host and the target host, the target responds with an RST packet to reset the connection. However, by doing so, it reveals its presence.

tcp fin scan

A FIN scan is initiated using a command like nmap -sF With so many different operating systems and versions around, it is really interesting how Nmap detects the operating system of a target in a very short time. Let us study the OS detection command in detail.

Security by obscurity? Assuming you are a Web developer, would you be interested in running an httpd service on a non standard TCP port — say, — rather than on the standard TCP port 80? In the early days, before I knew enough about various Nmap scan techniques, I thought this was just incredible! If a service is running on a nonstandard port, it does add a great layer of security.

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Welcome to the world of Nmap, which detects practically any service, even running on a non-standard port. The output is filtered. To specify every host individually is impractical, so you can specify this range in two different ways: Select the syntax that best suits your needs. A word of caution — take care when you use the CIDR notation. To verify which IP addresses will be scanned by the range you specified, add the -sL option the first time you run the command. This will only list all the hosts in the scan range, and will not initiate a scan.

After verifying the range, you can remove the -sL parameter.

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Please try out these scanning techniques, hands-on, before exploring further. Save my name, email, and website in this browser for the next time I comment.

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How harmful is it? Y Y asked. Medium Priority. Last Modified: Start Free Trial. View Solution Only. Commented: In what context? Greetings, LucF. Another technique sends erroneous packets at a port, expecting that "open" listening ports will send back different error messages than "closed" ports.

Nmap Tutorial for Security Professionals - TCP SYN Scan

The most common of these scans is the FIN scan, which attempts to close a connection that isn't open. If no service is listening at the target port, the operating system will generate an error message. If a service is listening, the operating system will silently drop the incoming packet. Therefore, no response indicates a listening service at the port. However, since packets can be dropped accidentally on the wire or by firewalls, this isn't a very effective scan. However, different operating systems respond differently to these scans.

Here is the description of a FIN scan from Insecure. Some firewalls and packet filters watch for SYNs to restricted ports, and programs like synlogger and Courtney are available to detect these scans.These three scan types even more are possible with the --scanflags option described in the next section exploit a subtle loophole in the TCP RFC to differentiate between open and closed ports.

Nmap exploits this with three scan types:. These three scan types are exactly the same in behavior except for the TCP flags set in probe packets. The key advantage to these scan types is that they can sneak through certain non-stateful firewalls and packet filtering routers. This configuration is common enough that the Linux iptables firewall command offers a special --syn option to implement it.

Another advantage is that these scan types are a little more stealthy than even a SYN scan. Don't count on this though—most modern IDS products can be configured to detect them. The big downside is that not all systems follow RFC to the letter. A number of systems send RST responses to the probes regardless of whether the port is open or not.

This causes all of the ports to be labeled closed. This scan does work against most Unix-based systems though. Since Nmap OS detection tests for this quirk, you can learn whether the scan works against a particular type of system by examining the nmap-os-db file. If the T2 line is longer, the system violated the RFC by sending a response and these scans won't work.

Another downside of these scans is that they can't distinguish open ports from certain filtered ones. But most filters simply drop banned probes without any response, making the ports appear open. Since Nmap cannot be sure which is the case, it marks non-responsive ports as open filtered. Adding version detection -sV can disambiguate as it does with UDP scans, but that defeats much of the stealthy nature of this scan.

If you are willing and able to connect to the ports anyway, you might as well use a SYN scan. Using these scan methods is simple. Just add the -sN-sFor -sX options to specify the scan type. The first one, a FIN scan against Para, identifies all five open ports as open filtered.

The next execution, an Xmas scan against scanme. It detects the closed port, but is unable to differentiate the filtered ports from the four open ones, all are listed as open filtered. This demonstrates why Nmap offers so many scan methods. No single technique is preferable in all cases. Ereet will simply have to try another method to learn more about Scanme. Demonstrating the full, firewall-bypassing power of these scans requires a rather lame target firewall configuration.

Unfortunately, those are easy to find.

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This example looks OK. Only two ports are open and the rest except for are filtered. With a modern stateful firewall, a FIN scan should not produce any extra information. That is a lot of apparently open ports. Most of them are probably open, because having just these 39 filtered and the other closed sending a RST packet would be unusual. Yet it is still possible that some or all are filtered instead of open.

FIN scan cannot determine for sure. We will revisit this case and learn more about Docsrv later in this chapter. Port Scanning Techniques and Algorithms.Novell is now a part of Micro Focus. Looking for Linux? See our new home at SUSE.

Before launching these attacks, the hackers engaged in some reconnaissance, or information-gathering, processes such as the following:. Social Engineering. By taking advantage of employees' unsuspecting nature, hackers can obtain important information about a company's network. For example, a hacker may pose as an executive's secretary and call the company's IS department, saying, "I'm Mr.

Markson's secretary. Unfortunately, Mr. Markson left his presentation on his desktop computer. I need his password to retrieve the file and send it to him.

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Of course, the hacker then writes down these users' passwords if those passwords are passed in clear text form. Security Leaks. If you lock your front doors but leave your windows open, you are susceptible to a break-in. Likewise, employees who leave confidential information in plain view on their desk or on white boards make their company susceptible to hackers. Employees may also make their company vulnerable by failing to secure sensitive information, such as passwords and access lists.

Scanning, Probing, and Listening. Using standard querying techniques and relying on their understanding of network communications and configurations, hackers actively and passively gather information about a company's network activity. This article and the accompanying Novell BrainShare presentation examines the types of scans hackers use to identify and characterize network devices. Specifically, this article focuses on the evidence that a scan has occurred--evidence that you can find by analyzing the packet-level communications that cross the wire.

After you understand the types of scans hackers use, you can build filters for your protocol analyzer to detect scans before hackers can actually launch a cyber attack. Before communicating with a host, an IP device must obtain the hardware address of the destination host or the next-hop router along the path to the host.

Hackers can discover active devices on the local network segment by sending a simple series of ARP broadcasts and incrementing the value for the target IP address field in each broadcast packet. For example, Figure 1 shows an ARP scan in progress.