*Encoder.Default then the default, Basic Latin only safelist will be used. See how our software enables the world to secure the web. 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. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. 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. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. Input validation. 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. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. If that isn't enough to keep in mind, you have to remember that encodings are lost when you retrieve them using the value attribute of a DOM element. . There will be times where you need to do something outside the protection provided by your framework. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. DOM XSS stands for Document Object Model-based Cross-site Scripting. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. These methods constitute the HTML Subcontext within the Execution Context. 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. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. 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. For example, you might need to close some existing elements before using your JavaScript payload. 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. If these methods are provided with untrusted input, then an XSS vulnerability could result. 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. However the opposite is the case with HTML encoding. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. These types of attacks typically occur as a result . Read more about DOM-based cross-site scripting. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. OWASP recommends DOMPurify for HTML Sanitization. Use a CSP as an additional layer of defense and have a look at the. //The following does NOT work because of the encoded "(" and ")". Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. Never rely on validation alone. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. The best way to fix DOM based cross-site scripting is to use the right output method (sink). The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. Validation can be a useful tool in limiting XSS attacks. Based on this context, you need to refine your input to see how it is processed. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. The #redir route is executed by another file, redir.html. Learn more about types of cross-site scripting attacks If this isn't possible, then ensure the data is JavaScript encoded. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. 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. For example, this is the case if you're loading a third-party library from a CDN. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. //The following DOES WORK because the encoded value is a valid variable name or function reference. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. Output encoding is not perfect. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Sometimes users need to author HTML. Finally, to fix the problem in our initial code, instead of trying to encode the output correctly which is a hassle and can easily go wrong we would simply use element.textContent to write it in a content like this: It does the same thing but this time it is not vulnerable to DOM based cross-site scripting vulnerabilities. Login here. Document Object Model (DOM) Based XSS. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. 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. 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). It also enables you to easily search your data without having to encode values before searching and allows you to take advantage of any changes or bug fixes made to encoders. 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. Get your questions answered in the User Forum. Encode all characters using the \xHH format. DOM-based cross-site scripting attack DOM-based XSS is also sometimes called "type-0 XSS." It occurs when the XSS vector executes as a result of a DOM modification on a website in a user's browser. Practise exploiting vulnerabilities on realistic targets. 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 (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. 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. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Validate all data that flows into your application from the server or a third-party API. This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against XSS. For DOM XSS, the attack is injected into the application during runtime in the client directly. 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. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. The reason why you only need to double JavaScript encode is that the customFunction function did not itself pass the input to another method which implicitly or explicitly called eval If firstName was passed to another JavaScript method which implicitly or explicitly called eval() then <%=doubleJavaScriptEncodedData%> above would need to be changed to <%=tripleJavaScriptEncodedData%>. How common is DOM-based cross-site scripting? The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. Please note, it is always dangerous design to put untrusted data directly into a command execution context. For example, websites often reflect URL parameters in the HTML response from the server. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). 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. Want to track your progress and have a more personalized learning experience? The following is an example vulnerability which occurs in the JavaScript context and HTML subcontext: Let's look at the individual subcontexts of the execution context in turn. See what Acunetix Premium can do for you. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. When other users load affected pages the attacker's scripts will run, enabling the attacker to steal cookies and session tokens, change the contents of the web page through DOM manipulation or redirect the browser to another page. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. DOM-based XSS Examples. A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. Sometimes you can't change the offending code. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. The HTML encoded value above is still executable. (It's free!). For that, first create a policy. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. Read the entire Acunetix Web Application Vulnerability Report. This means you will need to use alternative elements like img or iframe. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. 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 ">". // is an example of untrusted data that was properly JavaScript encoded but still executes. Each variable used in the user interface should be passed through an output encoding function. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. 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. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. How to prevent DOM-based cross-site scripting? DOM-Based Cross-Site Scripting. Record your progression from Apprentice to Expert. This fact makes it more difficult to maintain web application security. \u0064\u006f\u0063\u0075\u006d\u0065\u006e\u0074, \u0077\u0072\u0069\u0074\u0065\u006c\u006e, "\u0048\u0065\u006c\u006c\u006f\u0020\u0057\u006f\u0072\u006c\u0064", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0031\u0029", "url(<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(companyName))%>)", '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(userRelativePath))%>', "<%= Encode.forJavaScript(untrustedData) %>", "<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>", "customFunction('<%=doubleJavaScriptEncodedData%>', y)", //HTML encoding is happening in JavaScript, "javascript:myFunction('<%=untrustedData%>', 'test');", "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(untrustedData)) %>', 'test');",