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a) [3 marks] Describe the actions of the student using the web browser.

01 / 10 / 2021 Networking

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Question 1 - Protocol Analysis with Wireshark [8 marks]

Objective: gain a good understanding of common Internet protocols as well as using packet

capture software (Wireshark).

The file a01-assignment-1-question-1-capture.pcap on Moodle contains packets

captured in an exchange between several computers. Use the file to answer the following subquestions.

Some sub-questions refer to only a specific set of packets in the capture.

a) [3 marks] Considering all the packets in the file, draw a message sequence diagram that

illustrates the packets. A message sequence diagram uses vertical lines to represent

events that happen at a computer over time (time is increasing as the line goes down).

Addresses of the computers/software are given at the top of the vertical lines.

Horizontal or sloped arrows are used to show messages (packets) being sent between

computers. Each arrow should be labelled with the protocol, packet type and important

information of the message. An example of a portion of the message sequence diagram

for packets 6 and 7 are shown below. You must draw a diagram that shows all packets.

b) [5 marks] For the following sets of packets from the capture, give a short explanation

of their purpose/meaning. For example, why were they sent? Which computer sent

them? To which computer(s) did they send them to? What is the important information

in each packet? What is achieved from the packet exchange?

i. Packets 1 and 2

ii. Packets 3, 4 and 5

iii. Packets 6, 7, 8 and 9

iv. Packets 10 and 11

v. Packets 12, 13 and 14


Marking Scheme

Note: in this assignment the marking scheme gives indicators of what you need to achieve to

receive full or partial marks. Although usually only whole partial marks are listed, half (0.5)

marks may be given when appropriate.

a) A diagram is illustrated with correct order of packets and clearly labelled protocol, type

and any relevant information for each packet.

3 marks: the given diagram shows all packets, clearly and correctly labelled.

 2 marks: mostly correct but some minor mistakes

 1 mark: several mistakes that indicate partial understanding of how it works.

 0 marks: many mistakes that indicate not understanding how it works, or not


b) 1 mark per description. 1 mark if clear explanation of meaning of packets; 0.5 mark if

some mistakes or misunderstandings; 0 marks if wrong understanding of packets,

wrong packets described or many mistakes.

Question 2 – Application Attacks [14 marks]

Objective: be able to interpret packet captures and identify security weaknesses in web


You have performed a man-in-the-middle attack allowing you to capture packets between two

computers. The two computers are named/addressed as follows:

 Web Browser computer:

 Web Server computer:

The website is a university grading system: students and staff can log in and view grades of

courses they are involved with (staff or admin users can also edit grades). Students should not

be able to view other students’ grades, nor edit grades.

The packets you captured are in file a01-assignment-1-question-2-capture.pcap (on

Moodle). They involve a student, with student ID 5012345678, on the web browser computer

using the grading website. Unfortunately (for you) the actual login packets were encrypted

(using HTTPS) and therefore are not included in the file. Only the packets from after the login

are in the file.

By using only the file a01-assignment-1-question-2-capture.pcap and the information

in this question, answer the following sub-questions.

a) [3 marks] Describe the actions of the student using the web browser. That is, give the

links they clicked on (or typed in), the exact time when they clicked on it (relative to

the first packet captured), any parameters or form fields they filled in, and what was

displayed on their web browser as a result. You may give your answer as a list, for


- At time 0.0 the student clicked on link … . As a result they were shown a webpage

that welcomed them, had a link to “View grades”, “Logout” and to several help


- At time …

b) [2 marks] List all information you know about the student’s computer (including

addresses and software it uses). For each item of information, explain where you found

the item in the capture. For example, one item of information is the IP address:

- The student’s computer has IP address as identified in the IP header

of each packet.

- The student’s computer …

c) [3 marks] Explain how you could use the captured packets to perform an attack that

allows you to login as the student. In your answer, refer to the packets (and information

in them) that allow the attack and give a general explanation of how to perform the


d) [1 mark] Regarding the attack in the part (c), check if your favourite web browser allows

you to perform the attack. If so, explain what to do with that browser (e.g. “In Mozilla

Firefox on Windows 8.1, open the File menu and select …”). If not, then explain other

software/techniques you could use to perform the attack (e.g. “On Windows 8.1 install

the software … and then …”). Don’t forget to include the name of your favourite

browser in the answer. Note that you do not have to perform the steps – you only need

to find out what they are.

e) [3 marks] Now assume you have also obtained (through other means) the source code

of the file view.php in the grades directory on the web server. The file is on Moodle

and renamed as a01-assignment-1-question-2-view.php. Assuming your attack

from part (c) was successful and you are currently logged in as student 5012345678,

explain a different attack you can perform to view the grades of other students. In your

answer, explain what you would do to perform the attack and which lines of code in

view.php you take advantage of for the attack. Note that you can’t use the same attack

as part (c); your attack should make use of view.php.

f) [2 marks] Regarding the attack in part (d), explain how view.php could be modified

such that the attack would be unsuccessful.


Question 3 – Designing Firewalls [11 marks]

Objective: be able to design packet filtering firewall rules and identify

advantages/disadvantages of such firewalls.

An educational institute has a single router, referred to as the gateway router, connecting its

internal network to the Internet. The institute has the public address range and

the gateway router has address on its external interface (referred to as interface

ifext). The internal network consists of four subnets:

 A DMZ, which is attached to interface ifdmz of the gateway router and uses address


 A small network, referred to as shared, with interface ifint of the gateway router

connected to a second router, referred to as the internal router. This network has no

hosts attached (only two routers) and uses network address

 A staff subnet, which is for use by staff members only, that is attached to the internal

router and uses network address

 A student subnet, which is for use by students only, that is attached to the internal router

and uses network address

In summary, there are two routers in the network: the gateway router is attached to three subnets

(external, DMZ and shared); and the internal router is also attached to three subnets (shared,

staff and students).

There are three servers in the DMZ that all can accept requests from the Internet: a web server

supporting HTTP and HTTPS, a secure shell server using SSH, and a SMTP email server.

Internal staff and students can also access the web and secure shell servers; they can access the

email server but using IMAP.

The gateway router also runs a stateful packet filtering firewall and performs port address

translation. In addition to the DMZ setup as described above, security requirements for the

educational institute are:

1. External Internet users cannot access any internal computers (except in DMZ).

2. Both staff members and students can access websites in the Internet.

3. Staff, students and others may have accounts on the SSH server in the DMZ. However,

staff also have the additional requirement that if they login to the SSH server from the

Internet (e.g. while they are at home or travelling), they can then initiate a 2nd SSH

connection from the SSH server to their office computer on the staff subnet (this

assumes staff office computers also have SSH servers running). Staff can use this to

remotely access their office PCs from home. Students (and others outside the institute)

are not allowed to do this.

4. The Head of the institute finds performing two SSH connections to access their office

computer too difficult. They have requested that they be able to directly SSH from their

home computer to their office computer, bypassing the SSH server in the DMZ. The

Head has a fixed IP at home:

Considering the above information, answer the following questions:

a) [2 marks] Draw a diagram illustrating the network. Although there may be many

computers in the staff and student subnets, for simplicity you only have to draw three 

computers in the staff subnet and three computers in the student subnet. One of the three

in the staff subnet should be the Head’s office computer. Label all computers and router

interfaces with IP addresses.

b) [3 marks] Specify the firewall rules using the format as in the table below. You may

add/remove rows as needed. 

c) [3 marks] Give a short explanation of the rules. Also identify any potential issues with

the rules (that is, in practice what could go wrong when using your firewall rules) and

possible solutions.

d) [3 marks] Consider the Port Address Translation service on the gateway router. The

following table shows the addresses of packets arriving on interface ifint at the router.

The packets are listed in order at which they arrive at the router. Complete the addresses

for the packets as they are sent out on interface ifext of the router.


Marking Scheme

a) 2 marks if correct network is drawn and labelled. 1 mark if some mistakes in location

of nodes or links, or allocation of addresses. 0 marks if multiple mistakes in


b) and c) are marked together. 1 mark will be deducted for an incorrect rule or incorrect

explanation of the rule. 1 mark will be deducted for a missing rule. 1 mark will be

deducted for a rule that is included but not needed.

d) 0.5 mark if each row is correct. 

Question 4 – Vulnerability Research [7 marks]

Objective: research advanced malware and identify the basic techniques they are built upon.

New malware is being discovered regularly by security researchers. As the malware is

discovered, detection techniques are developed, but then malware developers improve their

malware to avoid detection. This cycle has led to very advanced malware, using a combination

of basic techniques. One widely publicised malware was Stuxnet. In 2010 Stuxnet was

discovered to have used various techniques to infect industrial control systems (which were not

connected to the Internet), with the possible purpose of shutting down facilities in Iran that

developed nuclear material. The techniques used have been linked to other attacks, and the

Equation Group.

Your task is to gain a basic understanding of how Stuxnet worked and recommend procedures

for avoiding similar types of advanced malware. A detailed explanation of Stuxnet is provided

by Symantec at:



This has sufficient information for you to learn about Stuxnet for this task, however you may

use other resources as well.

a) [3 marks] Summarise how Stuxnet worked. Refer to the concepts learnt in lectures in

your description (e.g. malware traits of circulation, infection, concealment, payload;

was it a logic bomb? did it use spam? …).

b) [4 marks] Provide a set of recommendations for how to avoid being impacted by

Stuxnet or similarly complex malware. For example, consider you are the Security

Manager in a company with large and important industrial control systems (e.g. power

companies, defence facilities, manufacturing) and you need to produce a concise list of

recommendations to avoid Stuxnet like attacks to the Chief Information Security

Officer. The recommendations should be justified, e.g. “Recommendation 1: do this

because of this reason”. The number of recommendations should be appropriate for the

number of different techniques Stuxnet uses.

Although there are no strict length requirements, part a) could be answered in ½ to 1 page,

while part b) could be answered with a list of multiple items with each item containing several


Marking Scheme

a) 3 marks if all or most of the key features are described at a level of detail that other

students could understand if they hadn’t read about Stuxnet. 2 marks if 1 or 2 of the key

features are not described or incorrectly described or have insufficient detail. 1 mark if

multiple features are incorrectly or not described. 0 marks if significant

misunderstanding of how Stuxnet works.

b) 4 marks if sufficient recommendations are given, with each clearly described and

justified. 3 marks of 1 or 2 important recommendations are missed or not clearly

described. 2 marks of 3 or 4 important recommendations are missed or not clearly

described. 0 or 1 mark if most of the recommendations are not relevant for Stuxnet or

not clearly described. 

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