You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Output encoding is the primary defense against cross-site scripting vulnerabilities. However, depending on the tag which innerText is applied, code can be executed. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. (It's free!). If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. However, if the pages returned from your web application utilize a content type of text/xhtml or the file type extension of *.xhtml then HTML encoding may not work to mitigate against XSS. See what Acunetix Premium can do for you. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. There are several methods and attributes which can be used to directly render HTML content within JavaScript. 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. Read the entire Acunetix Web Application Vulnerability Report. Since then, it has extended to include injection of basically any content, but we still refer to this as 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. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. DOM-based cross-site scripting happens when data from a user controlled, Most of the violations like this can also be detected by running a code linter or, If the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. 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. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Validate all data that flows into your application from the server or a third-party API. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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. Event handlers such as onload and onerror can be used in conjunction with these elements. 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. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). However, you may still find vulnerable code in the wild. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. Don't mutate DOM directly. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. 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. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Despite being rare, they may cause serious problems and only a few scanners can detect them. You should apply HTML attribute encoding to variables being placed in most HTML attributes. If your code looked like the following, you would need to only double JavaScript encode input data. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Sometimes users need to author HTML. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. Trusted Types force you to process a value. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. 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. DOM-based XSS simply means a cross-site scripting vulnerability that occurs in the DOM ( Document Object Model) of your site rather than in HTML. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. 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. Each variable in a web application needs to be protected. Websites may also store data on the server and reflect it elsewhere. Record your progression from Apprentice to Expert. 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. In JavaScript code, the main context is JavaScript but with the right tags and context closing characters, an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. The other alternative is using N-levels of encoding. Encoding libraries often have a EncodeForJavaScript or similar to support this function. 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:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. 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. Trusted Types force you to process a value somehow, but don't yet define what the exact processing rules are, and whether they are safe. One of the simplest ways of doing this is to deliver your exploit via an iframe: In this example, the src attribute points to the vulnerable page with an empty hash value. 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. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. We will look at eval, href and dangerouslySetHTML vulnerabilities. Free, lightweight web application security scanning for CI/CD. You need to work through each available source in turn, and test each one individually. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Each variable used in the user interface should be passed through an output encoding function. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. 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. your framework), you should be able to mitigate all XSS vulnerabilities. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. For example, when your application passes a string to innerHTML, the browser sends the following report: This says that in https://my.url.example/script.js on line 39 innerHTML was called with the string beginning with <img src=x. Use URL Encoding for these scenarios. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Download the latest version of Burp Suite. Learn more about types of cross-site scripting attacks Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . 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. If you directly access an encoder via System.Text.Encodings.Web. This is because these sinks treat the variable as text and will never execute it. If these methods are provided with untrusted input, then an XSS vulnerability could result. Please note, element.setAttribute is only safe for a limited number of attributes. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. No single technique will solve XSS. The primary rule that you must follow to prevent DOM XSS is: sanitize all untrusted data, even if it is only used in client-side scripts. Variables should not be interpreted as code instead of text. For each location where your string appears within the DOM, you need to identify the context. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. What would be displayed in the input text field would be "Johnson & Johnson". Now only JavaScript encoding on server side. DOM-based XSS Examples. In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. 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. Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. Sometimes you can't change the offending code. This behavior was often implemented using a vulnerable hashchange event handler, similar to the following: As the hash is user controllable, an attacker could use this to inject an XSS vector into the $() selector sink. 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. OWASP recommends these in all circumstances. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. Other CSS Contexts are unsafe and you should not place variable data in them. The attack functions by manipulating the internal model of the webpage within the browser known as the DOM and are referred to as DOM based attacks . 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). XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. Reduce the DOM XSS attack surface of your application. This brings up an interesting design point. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). Any variable that does not go through this process is a potential weakness. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. The world's #1 web penetration testing toolkit. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. These methods constitute the HTML Subcontext within the Execution Context. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. Save time/money. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. 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. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. Before putting untrusted data into a URL query string ensure it's URL encoded. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. 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. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. In many cases the context isn't always straightforward to discern. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Want to track your progress and have a more personalized learning experience? Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. DOM-based cross-site scripting is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. For instance, jQuery's attr() function can change the attributes of DOM elements. Some pure DOM-based vulnerabilities are self-contained within a single page. For example: To make dynamic updates to HTML in the DOM safe, we recommend: The HTML attribute subcontext within the execution context is divergent from the standard encoding rules. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. The DOM, or Document Object Model, is the structural format used to . You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. 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. Trusted Types require you to process the data before passing it to the above sink functions. Once you've found where the source is being read, you can use the JavaScript debugger to add a break point and follow how the source's value is used. 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. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Automatic encoding and escaping functions are built into most frameworks. For that, first create a policy. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. Content Security Policy - An allowlist that prevents content being loaded. At a basic level XSS works by tricking your application into inserting a