Consider adopting the following controls in addition to the above. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Input validation. DOM-based XSS is a kind of XSS occurring entirely on the client-side. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). //The following does NOT work because of the encoded "(" and ")". 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. Already got an account? Use a CSP as an additional layer of defense and have a look at the. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. This is because these sinks treat the variable as text and will never execute it. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. If you directly access an encoder via System.Text.Encodings.Web. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. There are two ways to do this. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. HTML Context refers to inserting a variable between two basic HTML tags like a
or . How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. There are a couple of options for fixing a Trusted Type violation. How to prevent DOM-based cross-site scripting? . Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. 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. Using the right combination of defensive techniques is necessary to prevent XSS. Output encoding is not perfect. 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. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. DOM-based XSS attacks seek to exploit the DOM in a simple two step process: Create a Source: Inject a malicious script into a property found to be suceptible to DOM-based XSS attacks. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). This fact makes it more difficult to maintain web application security. The only safe location for placing variables in JavaScript is inside a quoted data value. 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. //The following DOES WORK because the encoded value is a valid variable name or function reference. In those cases, create a Trusted Type object yourself. Learn the details here including XSS prevention methods. For example, if your string appears within a double-quoted attribute then try to inject double quotes in your string to see if you can break out of the attribute. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Any variable that does not go through this process is a potential weakness. For example, Acunetix. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. Catch critical bugs; ship more secure software, more quickly. The web application dynamically generates a web page that contains this untrusted data. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Thankfully, many sinks where variables can be placed are safe. For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. URL Contexts refer to variables placed into a URL. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. element.SetAttribute () element [attribute]= At a basic level XSS works by tricking your application into inserting a