Matt Posted August 4, 2013 Report Posted August 4, 2013 What is DOM?DOM is expanded as Document object model that allows client-side-scripts(Eg: Javascript) to dynamically access and modify the content, structure, and style of a webpage.Like server-side scripts, client-side scripts can also accept and manipulate user input with the help of DOM.Here is a very simple HTML code that accepts and writes user input using JavaScript with the help of DOM.<html> <head></head><body> <script>var pos=document.URL.indexOf("BTSinput=")+9; //finds the position of value var userInput=document.URL.substring(pos,document.URL.length); //copy the value into userInput variabledocument.write(unescape(userInput)); //writes content to the webpage </script></body></html>If you know HTML and Javscript, understanding the above code is a piece of cake.In the above example, the javascript code gets value from the url parameter "BTSinput" and writes the value in our webpage.For example, if the url iswww.BreakThesecurity.com/PenTesting?BTSinput=defaultThe webpage will display "default" as output.Did you notice ?! The part of the webpage is not written by Server-side script. The client side script modifies the content dynamically based on the input. Everything done with the help of DOM object 'document'.DOM Based XSS vulnerability:When a developer writes the content using DOM object without sanitizing the user input , it allow an attacker to run his own code. In above example, we failed to sanitize the input and simply displayed the whatever value we get from the url. An attacker with malicious intention can inject a xss vector instead . For example:www.BreakThesecurity.com/PenTesting?BTSinput=<script>alert("BreakTheSec")</script>As i said earlier, the document.write function simply writes the value of BTSinput parameter in the webpage. So it will write the '<script>alert("BreakTheSec")</script>' in the webpage without sanitizing. This results in running the script code and displays the alert box.Patching the DOM Based Cross Site Scripting VulnerabilityAudit all JavaScript code in use by your application to make sure that untrusted data is being escaped before being written into the document, evaluated, or sent as part of an AJAX request. There are dozens of JavaScript functions and properties which must be protected, including some which are rather non-obvious:The document.write() functionThe document.writeln() functionThe eval() function, which executes JavaScript code from a stringThe execScript() function, which works similarly to eval()The setInterval(), setTimeout(), and navigate() functionsThe .innerHTML property of a DOM elementCertain CSS properties which allow URLs such as .style, .backgroundImage, .listStyleImage, etc.The event handler properties like .onClick, which take JavaScript code as their valuesAny data which is derived from data under the client's control (e.g. request parameters, headers, query parameters, cookie names and values, the URL of the request itself, etc.) should be escaped before being used. Examples of user-controlled data include document.location (and most of its properties, e.g. document.location.search), document.referrer, cookie names and values, and request header names and values.You can use the JavaScript built-in functions encode() or encodeURI() to handle your escaping. If you write your own escaping functions, be extremely careful. Rather than using a "black list" approach (where you filter dangerous characters and pass everything else through untouched), it is better to use a "white list" approach. A good white list approach is to escape everything by default and allow only alphanumeric characters through.Reference:Vulnerability & Exploit Database | Rapid7 Sursa Breakthesecurity.com Quote
Active Members dancezar Posted August 4, 2013 Active Members Report Posted August 4, 2013 Mai adaug si eu ceva.Mai este si acesta un tip de DOOM XSSacesta este scriptul vurnerabil<script>document.write(document.location.hash);</script>site.com/script_vurnerabil.html#<script>alert(1)</script> Quote
