The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. 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. An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. For example, this is the case if you're loading a third-party library from a CDN. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. The example that follows illustrates using closures to avoid double JavaScript encoding. 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. The line above could have possibly worked to render a link. Before putting untrusted data inside an HTML element ensure it's HTML encoded. 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. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. 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%>. Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. Get help and advice from our experts on all things Burp. Read about other types of cross-site scripting attacks. 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 . One example of an attribute which is thought to be safe is innerText. 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. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. A list of safe HTML attributes is provided in the Safe Sinks section. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. 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. Summary. There will be situations where you use a URL in different contexts. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). The only safe location for placing variables in JavaScript is inside a quoted data value. Level up your hacking and earn more bug bounties. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Normally executing JavaScript from a CSS context required either passing javascript:attackCode() to the CSS url() method or invoking the CSS expression() method passing JavaScript code to be directly executed. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. It is an informational message with a simple alert. There may be times you want to insert a value into JavaScript to process in your view. However, depending on the tag which innerText is applied, code can be executed. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Use URL Encoding for these scenarios. 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. // is an example of untrusted data that was properly JavaScript encoded but still executes. As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. 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. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Variables should not be interpreted as code instead of text. This is why you would need to HTML encode too. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. For example: Modern web applications are typically built using a number of third-party libraries and frameworks, which often provide additional functions and capabilities for developers. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. The other alternative is using N-levels of encoding. 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. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Cross-Site Scripting (XSS) is a misnomer. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). OWASP recommends these in all circumstances. What's the difference between Pro and Enterprise Edition? If you have to use user input on your page, always use it in the text context, never as HTML tags or any other potential code. The third cross site scripting attack occurs entirely in the browser. The DOM is a programming interface. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). 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. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. DOM-based XSS is a kind of XSS occurring entirely on the client-side. I will show you three examples of DOM-based XSS attacks in this article. In these cases, HTML Sanitization should be used. Always pass untrusted input as a query string value. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. 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 sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Event handlers such as onload and onerror can be used in conjunction with these elements. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. Don't mutate DOM directly. These types of attacks typically occur as a result . 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. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. The rendered output would now become. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. //The following does NOT work because the event handler is being set to a string. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. This document only discusses JavaScript bugs which lead to XSS. 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. HTML Context refers to inserting a variable between two basic HTML tags like a