Get the latest content on web security in your inbox each week. As with all other Cross-site Scripting (XSS) vulnerabilities, this type of attack also relies on insecure handling of user input on an HTML page. Don't use untrusted input as part of a URL path. This view outputs the contents of the untrustedInput variable. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. placed in an HTML Attribute. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Now only JavaScript encoding on server side. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. 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. 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. //The following does NOT work because the event handler is being set to a string. A script within the later response contains a sink which then processes the data in an unsafe way. This can be done via a function such as: For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Even newer versions of jQuery can still be vulnerable via the $() selector sink, provided you have full control over its input from a source that doesn't require a # prefix. These methods constitute the HTML Subcontext within the Execution Context. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The primary difference is where the attack is injected into the application. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. Examples of safe attributes includes: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. Enhance security monitoring to comply with confidence. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. Free, lightweight web application security scanning for CI/CD. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. In some . This brings up an interesting design point. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. The best manual tools to start web security testing. The #redir route is executed by another file, redir.html. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. //The following does NOT work because of the encoded "(" and ")". To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. This fact makes it more difficult to maintain web application security. This is a Safe Sink and will automatically CSS encode data in it. Variables should not be interpreted as code instead of text. This could lead to an attack being added to a webpage.. for example. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Here are some examples of encoded values for specific characters. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. If you're using JavaScript to change a CSS property, look into using style.property = x. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. If you must, the following examples describe some approaches that do and do not work. This cheat sheet provides guidance to prevent XSS vulnerabilities. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. XSS Prevention & Mitigation. . Each encoder, Html, JavaScript and Url, must be configured separately. Doing so encourages designs in which the security rules are close to the data that they process, where you have the most context to correctly sanitize the value. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Practise exploiting vulnerabilities on realistic targets. Trusted Types work by locking down the following risky sink functions. This means you will need to use alternative elements like img or iframe. 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. DOM-Based Cross-Site Scripting. Want to track your progress and have a more personalized learning experience? Web Application Firewalls - These look for known attack strings and block them. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. 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. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. . 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. We will look at eval, href and dangerouslySetHTML vulnerabilities. Any application is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can develop from source to sink. There are a variety of sinks that are relevant to DOM-based vulnerabilities. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. HTML encoding takes characters such as < and changes them into a safe form like < Before putting untrusted data into an HTML attribute ensure it's HTML encoded. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. See what Acunetix Premium can do for you. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Encode all characters using the \xHH format. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". Read the entire Acunetix Web Application Vulnerability Report. Already got an account? Use only safe functions like document.innerText and document.textContent. OWASP recommends DOMPurify for HTML Sanitization. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. This is because these sinks treat the variable as text and will never execute it. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. For example, Acunetix. WAFs also dont address the root cause of an XSS vulnerability. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. 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. Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. Some pure DOM-based vulnerabilities are self-contained within a single page. The Unicode standard has a list of code charts you can use to find the chart containing your characters. All other contexts are unsafe and you should not place variable data in them. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. Trusted Types heavily reduce the DOM XSS attack surface of your application. Make sure any attributes are fully quoted, same as JS and CSS. It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. By default encoders use a safe list limited to the Basic Latin Unicode range and encode all characters outside of that range as their character code equivalents. The problem is that if companyName had the value "Johnson & Johnson". //The following DOES WORK because the encoded value is a valid variable name or function reference. Output Encoding and HTML Sanitization help address those gaps. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. Information on ordering, pricing, and more. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Please insert your password to refresh your session. Please note, it is always dangerous design to put untrusted data directly into a command execution context. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. The HTML parser of the rendering context dictates how data is presented and laid out on the page and can be further broken down into the standard contexts of HTML, HTML attribute, URL, and CSS. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. How to prevent DOM-based cross-site scripting? Get your questions answered in the User Forum. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. Note that browsers behave differently with regards to URL-encoding, Chrome, Firefox, and Safari will URL-encode location.search and location.hash, while IE11 and Microsoft Edge (pre-Chromium) will not URL-encode these sources. 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. Parsing HTML input is difficult, if not impossible. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. For DOM XSS, the attack is injected into the application during runtime in the client directly. 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.

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