The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. RULE #1 - HTML Escape then JavaScript Escape Before Inserting Untrusted Data into HTML Subcontext within the Execution Context, RULE #2 - JavaScript Escape Before Inserting Untrusted Data into HTML Attribute Subcontext within the Execution Context, RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context, RULE #4 - JavaScript Escape Before Inserting Untrusted Data into the CSS Attribute Subcontext within the Execution Context, RULE #5 - URL Escape then JavaScript Escape Before Inserting Untrusted Data into URL Attribute Subcontext within the Execution Context, RULE #6 - Populate the DOM using safe JavaScript functions or properties, RULE #7 - Fixing DOM Cross-site Scripting Vulnerabilities, Guidelines for Developing Secure Applications Utilizing JavaScript, GUIDELINE #1 - Untrusted data should only be treated as displayable text, GUIDELINE #2 - Always JavaScript encode and delimit untrusted data as quoted strings when entering the application when building templated JavaScript, GUIDELINE #3 - Use document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar to build dynamic interfaces, GUIDELINE #4 - Avoid sending untrusted data into HTML rendering methods, GUIDELINE #5 - Avoid the numerous methods which implicitly eval() data passed to it, Utilizing an Enclosure (as suggested by Gaz), GUIDELINE #6 - Use untrusted data on only the right side of an expression, GUIDELINE #7 - When URL encoding in DOM be aware of character set issues, GUIDELINE #8 - Limit access to object properties when using object[x] accessors, GUIDELINE #9 - Run your JavaScript in a ECMAScript 5 canopy or sandbox, GUIDELINE #10 - Don't eval() JSON to convert it to native JavaScript objects, Common Problems Associated with Mitigating DOM Based XSS, Insecure Direct Object Reference Prevention, Creative Commons Attribution 3.0 Unported License. Free, lightweight web application security scanning for CI/CD. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. 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. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. XSS sinks are places where variables are placed into your webpage. This is because these sinks treat the variable as text and will never execute it. 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. DOM-based XSS is an advanced XSS attack. DOM-based XSS Examples. DOM based cross site scripting (Video solution) - YouTube Validation becomes more complicated when accepting HTML in user input. (It's free!). The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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=. The styling will not be rendered. Please note, element.setAttribute is only safe for a limited number of attributes. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Reduce the DOM XSS attack surface of your application. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. If you're using JavaScript for writing to HTML, look at the .textContent attribute as it is a Safe Sink and will automatically HTML Entity Encode. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. What is cross-site scripting (XSS) and how to prevent it? | Web Sometimes you can't change the offending code. It is also impossible to protect against such client-side attacks using WAFs. // is an example of untrusted data that was properly JavaScript encoded but still executes. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. When a browser is rendering HTML and any other associated content like CSS or JavaScript, it identifies various rendering contexts for the different kinds of input and follows different rules for each context. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. React XSS Cross-site scripting prevention - Dev Academy DOMPurify supports Trusted Types and will return sanitized HTML wrapped in a TrustedHTML object such that the browser does not generate a violation.CautionIf the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. CWE - CWE-79: Improper Neutralization of Input During Web Page It is the process of converting untrusted . If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. Prevent Cross-Site Scripting (XSS) in ASP.NET Core For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. There are some further things to consider: Security professionals often talk in terms of sources and sinks. Customization of the safe list only affects encoders sourced via DI. Output encoding is not perfect. The web application dynamically generates a web page that contains this untrusted data. In these cases, HTML Sanitization should be used. These methods constitute the HTML Subcontext within the Execution Context. Automatic encoding and escaping functions are built into most frameworks. What is WordPress Cross-site Scripting (XSS) and How to prevent it? How to prevent DOM-based cross-site scripting? Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. Developers should use the following prevention steps to avoid introducing XSS into their application. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. Types of XSS (Cross-site Scripting) - Acunetix Scale dynamic scanning. element.SetAttribute () element [attribute]= This is a Safe Sink and will automatically URL encode data in it. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. That said, developers need to be aware of problems that can occur when using frameworks insecurely such as: Understand how your framework prevents XSS and where it has gaps. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. The #redir route is executed by another file, redir.html. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. These locations are known as dangerous contexts. What is DOM-based cross-site scripting? - PortSwigger Common injection vectors include document.url, document.location, and document.referrer objects. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. How to Prevent DOM-based Cross-site Scripting - blackMORE Ops It's important to remember that some of these are also potential sources and sinks for DOM XSS. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. DOM-based XSS is a kind of XSS occurring entirely on the client-side. 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. Safe list ranges are specified as Unicode code charts, not languages. We want to hear from you! In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. DOM-based cross-site scripting is the de-facto name for XSS bugs that are the result of active browser-side content on a page, typically JavaScript, obtaining user input and then doing something unsafe with it, leading to the execution of injected code. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. If A is double JavaScript encoded then the following if check will return false. OWASP TOP 10: Cross-site scripting (XSS) ~2023 | Udemy One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. It uses HTML attribute encoding rules whenever you use the @ directive. The encoder safe lists can be customized to include Unicode ranges appropriate to the app during startup, in Program.cs: For example, using the default configuration using a Razor HtmlHelper similar to the following: The preceding markup is rendered with Chinese text encoded: To widen the characters treated as safe by the encoder, insert the following line into Program.cs. DOM-based XSS Vulnerability - All you need to know - Crashtest Security At a basic level XSS works by tricking your application into inserting a