Some examples of DOM-based XSS attacks include: 1. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. your framework), you should be able to mitigate all XSS vulnerabilities. Output encoding is the primary defense against cross-site scripting vulnerabilities. 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. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . For that, first create a policy. How to detect DOM-based cross-site scripting? We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. In some . Get started with Burp Suite Professional. 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. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. 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. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. 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. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. Get help and advice from our experts on all things Burp. 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. If these methods are provided with untrusted input, then an XSS vulnerability could result. If you're using JavaScript to change a CSS property, look into using style.property = x. 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. 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. Encoding libraries often have a EncodeForJavaScript or similar to support this function. If A is double JavaScript encoded then the following if check will return false. You need to work through each available source in turn, and test each one individually. 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). Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. XSS is one of the most common and dangerous web vulnerabilities, and it is . Use URL Encoding for these scenarios. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. On the client side, the HTTP response does not change but the script executes in malicious manner. 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. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. There are a couple of options for fixing a Trusted Type violation. 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(). 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. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. The world's #1 web penetration testing toolkit. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. In this section, we'll describe DOM-based cross-site scripting (DOM XSS), explain how to find DOM XSS vulnerabilities, and talk about how to exploit DOM XSS with different sources and sinks. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. Record your progression from Apprentice to Expert. 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. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. We are looking for web developers to participate in user research, product testing, discussion groups and more. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. 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. Also, XSS attacks always execute in the browser. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. There are several methods and attributes which can be used to directly render HTML content within JavaScript. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. Encode all characters with the %HH encoding format. Learn more about types of cross-site scripting attacks placed in an HTML Attribute. This brings up an interesting design point. 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. 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. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. Get the latest content on web security in your inbox each week. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. 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. 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. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. 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. Sometimes users need to author HTML. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. A list of safe HTML attributes is provided in the Safe Sinks section. Some pure DOM-based vulnerabilities are self-contained within a single page. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. For DOM XSS, the attack is injected into the application during runtime in the client directly. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. For example, you might need to close some existing elements before using your JavaScript payload. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Before putting untrusted data inside an HTML element ensure it's HTML encoded. A script within the later response contains a sink which then processes the data in an unsafe way. The web application dynamically generates a web page that contains this untrusted data. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. . In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template. 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 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. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. Trusted Types require you to process the data before passing it to the above sink functions. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. Cross-Site Scripting (XSS) is a misnomer. //any code passed into lName is now executable. We want to hear from you! The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. . A Computer Science portal for geeks. No single technique will solve XSS. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. Customization of the safe list only affects encoders sourced via DI. 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. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. DOM based XSS vulnerabilities therefore have to be prevented on the client side. Misconceptions abound related to the proper encoding that is required. //The following does NOT work because of the encoded ";". The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. 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. eval For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Do your applications use this vulnerable package? To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. This is common when you want users to be able to customize the look and feel of their webpages. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). jQuery used to be extremely popular, and a classic DOM XSS vulnerability was caused by websites using this selector in conjunction with the location.hash source for animations or auto-scrolling to a particular element on the page. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. The line above could have possibly worked to render a link. The following are some of the main sinks that can lead to DOM-XSS vulnerabilities: The following jQuery functions are also sinks that can lead to DOM-XSS vulnerabilities: In addition to the general measures described on the DOM-based vulnerabilities page, you should avoid allowing data from any untrusted source to be dynamically written to the HTML document. - owasp-CheatSheetSeries . The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. 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. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. 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. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. 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. The HTML encoded value above is still executable. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. 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. CSS Contexts refer to variables placed into inline CSS. If you must, the following examples describe some approaches that do and do not work. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Input validation. document.CreateTextNode () and append it in the appropriate DOM location. 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. There are two distinct groups of cross-site scripting. Get the latest content on web security in your inbox each week. 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. . Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. However, you may still find vulnerable code in the wild. 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%>. 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. Each encoder, Html, JavaScript and Url, must be configured separately. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. More info about Internet Explorer and Microsoft Edge. 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. 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. Avoid treating untrusted data as code or markup within JavaScript code. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. 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. Developers should use the following prevention steps to avoid introducing XSS into their application. Avoid populating the following methods with untrusted data. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks.
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