Variables should only be placed in a CSS property value. 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. //The following DOES WORK because the encoded value is a valid variable name or function reference. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Read the entire Acunetix Web Application Vulnerability Report. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. How common is DOM-based cross-site scripting? DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Get the latest content on web security in your inbox each week. Then client-side encode (using a JavaScript encoding library such as node-esapi) for the individual subcontext (DOM methods) which untrusted data is passed to. It is a simple yet effective way to harvest passwords using only the victims browser. Event handlers such as onload and onerror can be used in conjunction with these elements. 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. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. The most common one would be adding it to an href or src attribute of an tag. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. It uses HTML attribute encoding rules whenever you use the @ directive. There are a couple of options for fixing a Trusted Type violation. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. OWASP recommends these in all circumstances. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. WAFs also dont address the root cause of an XSS vulnerability. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. 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. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". XSS is one of the most common and dangerous web vulnerabilities, and it is . Using untrusted user data on the left side of the expression allows an attacker to subvert internal and external attributes of the window object, whereas using user input on the right side of the expression doesn't allow direct manipulation. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. This is a Safe Sink and will automatically CSS encode data in it. You need to work through each available source in turn, and test each one individually. Reduce risk. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. The line above could have possibly worked to render a link. Don't use untrusted input as part of a URL path. Use only safe functions like document.innerText and document.textContent. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. 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=. Web Application Firewalls - These look for known attack strings and block them. Learn the details here including XSS prevention methods. Encode all characters using the \xHH format. 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. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. 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. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Perhaps the non-conforming functionality is not needed anymore or can be rewritten in a modern way without using the error-prone functions?Don'tel.innerHTML = '<img src=xyz.jpg>'; Doel.textContent = '';const img = document.createElement('img');img.src = 'xyz.jpg';el.appendChild(img); Some libraries already generate Trusted Types that you can pass to the sink functions. HTML tag elements are well defined and do not support alternate representations of the same tag. 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. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Common injection vectors include document.url, document.location, and document.referrer objects. Using the right combination of defensive techniques is necessary to prevent XSS. At a basic level XSS works by tricking your application into inserting a