Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. Working example (no HTML encoding): Normally encoded example (Does Not Work DNW): HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW): If HTML encoding followed the same semantics as JavaScript encoding. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. If you pollute a river, it'll flow downstream somewhere. There are a couple of options for fixing a Trusted Type violation. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. You should apply HTML attribute encoding to variables being placed in most HTML attributes. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. Cross-Site Scripting (XSS) is a misnomer. For example if you want to use user input to write in a div tag element don't use innerHtml, instead use innerText or textContent. Understanding the XSS Threat: A Comprehensive Guide to DOM Based Cross If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. Validate all data that flows into your application from the server or a third-party API. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. It is the process of converting untrusted . eval Save time/money. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. Some examples of DOM-based XSS attacks include: 1. Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn't properly filter user input. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. The styling will not be rendered. How common is DOM-based cross-site scripting? Each encoder, Html, JavaScript and Url, must be configured separately. Use a trusted and verified library to escape HTML inputs. Please refer to the list below for details. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Cross-site scripting XSS Event handlers such as onload and onerror can be used in conjunction with these elements. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. Input validation. Testing JavaScript execution sinks for DOM-based XSS is a little harder. For example, Acunetix. DOM-based XSS: DOM-based XSS occurs when an . This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. . //The following does NOT work because of the encoded ";". These locations are known as dangerous contexts. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. This is because these sinks treat the variable as text and will never execute it. This is why you would need to HTML encode too. Read more about DOM-based cross-site scripting. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. Therefore, the primary recommendation is to avoid including untrusted data in this context. If you sanitize content and then modify it afterwards, you can easily void your security efforts. Developers should use the following prevention steps to avoid introducing XSS into their application. A list of output encoding libraries is included in the appendix. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. Ideally, the correct way to apply encoding and avoid the problem stated above is to server-side encode for the output context where data is introduced into the application. If you need to render different content, use innerText instead of innerHTML. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. Now only JavaScript encoding on server side. HTML Context refers to inserting a variable between two basic HTML tags like a