There are two ways to do this. Script manipulation: <script src> and setting text content of <script> elements. Fewer XSS bugs appear in applications built with modern web frameworks. For instance, jQuery's attr() function can change the attributes of DOM elements. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Safe list ranges are specified as Unicode code charts, not languages. 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. So XSS has already been around for a while. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. The line above could have possibly worked to render a link. Get the latest content on web security in your inbox each week. If you need to render different content, use innerText instead of innerHTML. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. 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. The other alternative is using N-levels of encoding. The primary difference is where the attack is injected into the application. Summary. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. 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. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. In the case above, JavaScript encoding does not mitigate against DOM based XSS. 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 . The HTML encoded value above is still executable. Validation can be a useful tool in limiting XSS attacks. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. 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. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Make sure any attributes are fully quoted, same as JS and CSS. The Unicode standard has a list of code charts you can use to find the chart containing your characters. Encode all characters using the \xHH format. What would be displayed in the input text field would be "Johnson & Johnson". These locations are known as dangerous contexts. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Variables should not be interpreted as code instead of text. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. What's the difference between Pro and Enterprise Edition? Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. 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. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. The complication is compounded by the differing meanings and treatment of encoded values within each subcontext (HTML, HTML attribute, URL, and CSS) within the execution context. Use only safe functions like document.innerText and document.textContent. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. . A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. Any variable that does not go through this process is a potential weakness. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. 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. Want to track your progress and have a more personalized learning experience? These types of attacks typically occur as a result . To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. 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. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. This is common when you want users to be able to customize the look and feel of their webpages. 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. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. After the page's JavaScript applies this malicious URL to the back link's href, clicking on the back link will execute it: Another potential sink to look out for is jQuery's $() selector function, which can be used to inject malicious objects into the DOM. 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. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. Websites may also store data on the server and reflect it elsewhere. The name originated from early versions of the attack where stealing data cross-site was the primary focus. If you pollute a river, it'll flow downstream somewhere. This is because these sinks treat the variable as text and will never execute it. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. All other contexts are unsafe and you should not place variable data in them. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. 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. Document Object Model (DOM) Based XSS. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. If this isn't possible, then ensure the data is JavaScript encoded. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). The logic which parses URLs in both execution and rendering contexts looks to be the same. The only safe location for placing variables in JavaScript is inside a quoted data value. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. 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. XSS sinks are places where variables are placed into your webpage. For example. Sometimes you can't change the offending code. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. WAFs are unreliable and new bypass techniques are being discovered regularly. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. 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. Get started with Burp Suite Professional. One example of an attribute which is thought to be safe is innerText. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. Trusted Types work by locking down the following risky sink functions. 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. Event handlers such as onload and onerror can be used in conjunction with these elements.

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