The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Therefore, the primary recommendation is to avoid including untrusted data in this context. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. For example, you might need to close some existing elements before using your JavaScript payload. Now only JavaScript encoding on server side. Thankfully, many sinks where variables can be placed are safe. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. This means, that no data will be available in server logs. Trusted Types work by locking down the following risky sink functions. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. 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. 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. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. DOM Based Attacks. To test for DOM XSS in an HTML sink, place a random alphanumeric string into the source (such as location.search), then use developer tools to inspect the HTML and find where your string appears. Variables should only be placed in a CSS property value. For each location where your string appears within the DOM, you need to identify the context. Cross-Site Scripting (XSS) is a misnomer. Use a CSP as an additional layer of defense and have a look at the. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. DOM based XSS vulnerabilities therefore have to be prevented on the client side. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Read the entire Acunetix Web Application Vulnerability Report. If your code looked like the following, you would need to only double JavaScript encode input data. This view outputs the contents of the untrustedInput variable. Output Encoding. DOM Based XSS | OWASP Foundation //The following does NOT work because of the encoded ";". HTML tag elements are well defined and do not support alternate representations of the same tag. For DOM XSS, the attack is injected into the application during runtime in the client directly. If you must, the following examples describe some approaches that do and do not work. XSS sinks are places where variables are placed into your webpage. DOM-based XSS: DOM-based XSS occurs when an . Each parser has distinct and separate semantics in the way they can possibly execute script code which make creating consistent rules for mitigating vulnerabilities in various contexts difficult. Any variable that does not go through this process is a potential weakness. In some . DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. The third cross site scripting attack occurs entirely in the browser. React XSS Guide: Examples and Prevention - StackHawk Misconceptions abound related to the proper encoding that is required. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. See how our software enables the world to secure the web. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. See what Acunetix Premium can do for you. How to prevent DOM-based cross-site scripting? To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: createElement () and assign property values with appropriate methods or properties such as node.textContent= or node.InnerText=. Use URL Encoding for these scenarios. There are a couple of options for fixing a Trusted Type violation. Some examples of DOM-based XSS attacks include: 1. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. Safe HTML Attributes include: 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. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. We are looking for web developers to participate in user research, product testing, discussion groups and more. This document only discusses JavaScript bugs which lead to XSS. Understanding the XSS Threat: A Comprehensive Guide to DOM Based Cross Always pass untrusted input as a query string value. Its critical to use quotation marks like " or ' to surround your variables. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. At a basic level XSS works by tricking your application into inserting a