The best way to fix DOM based cross-site scripting is to use the right output method (sink). The best manual tools to start web security testing. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. A script within the later response contains a sink which then processes the data in an unsafe way. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. You need to work through each available source in turn, and test each one individually. CSS is surprisingly powerful and has been used for many types of attacks. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. The document.write sink works with script elements, so you can use a simple payload, such as the one below: Note, however, that in some situations the content that is written to document.write includes some surrounding context that you need to take account of in your exploit. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. What is Cross-Site Scripting (XSS) and How to Prevent It? For each location where your string appears within the DOM, you need to identify the context. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: createElement () and assign property values with appropriate methods or properties such as node.textContent= or node.InnerText=. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. See Browser compatibility for up-to-date cross-browser support information.Key TermDOM-based cross-site scripting happens when data from a user controlled source (like user name, or redirect URL taken from the URL fragment) reaches a sink, which is a function like eval() or a property setter like .innerHTML, that can execute arbitrary JavaScript code. Catch critical bugs; ship more secure software, more quickly. Stored XSS is considered the most damaging type of XSS attack. When this happens, a script on the web page selects the URL variable and executes the code it contains. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. 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. 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. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. Safe list ranges are specified as Unicode code charts, not languages. Want to track your progress and have a more personalized learning experience? DOM-based XSS Examples. In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template. If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. Cross Site Scripting Prevention Cheat Sheet - github.com DOM-based XSS: DOM-based XSS occurs when an . CWE - CWE-79: Improper Neutralization of Input During Web Page Trusted Types give you the tools to write, security review, and maintain applications free of DOM XSS vulnerabilities by making the dangerous web API functions secure by default. DOM based cross site scripting (Video solution) - YouTube Use a trusted and verified library to escape HTML inputs. DOM-based XSS vulnerabilities usually arise when JavaScript takes data from an attacker-controllable source, such as the URL, and passes it to a sink that supports dynamic code execution, such as eval () or innerHTML. 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. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. This is why you would need to HTML encode too. Semgrep rule to identify above dom xss link. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. The example that follows illustrates using closures to avoid double JavaScript encoding. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. For example, Acunetix. Also, XSS attacks always execute in the browser. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. This can be done via a function such as: The only safe location for placing variables in JavaScript is inside a quoted data value. . View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. javascript - XSS prevention and .innerHTML - Stack Overflow Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. In some . DOM XSS: An Explanation of DOM-based Cross-site Scripting The data is subsequently read from the DOM by the web application and outputted to the browser. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. Sometimes users need to author HTML. Cross-Site Scripting (XSS) is a misnomer. At a basic level XSS works by tricking your application into inserting a