DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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 . An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. Sometimes users need to author HTML. Accelerate penetration testing - find more bugs, more quickly. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. //The following DOES WORK because the encoded value is a valid variable name or function reference. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. We are looking for web developers to participate in user research, product testing, discussion groups and more. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. The other alternative is using N-levels of encoding. 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. 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. //The following does NOT work because of the encoded ";". Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Document Object Model (DOM) Based XSS. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. DOM-based XSS is an advanced XSS attack. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. In many cases the context isn't always straightforward to discern. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. Validation can be a useful tool in limiting XSS attacks. For example.. An attacker could modify data that is rendered as $varUnsafe. You should apply HTML attribute encoding to variables being placed in most HTML attributes. OWASP recommends DOMPurify for HTML Sanitization. Always pass untrusted input as a query string value. 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. The encoder safe lists can be customized to include Unicode ranges appropriate to the app during startup, in Program.cs: For example, using the default configuration using a Razor HtmlHelper similar to the following: The preceding markup is rendered with Chinese text encoded: To widen the characters treated as safe by the encoder, insert the following line into Program.cs. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. The world's #1 web penetration testing toolkit. There will be situations where you use a URL in different contexts. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. This cheat sheet provides guidance to prevent XSS vulnerabilities. A list of output encoding libraries is included in the appendix. This is where Output Encoding and HTML Sanitization are critical. The example that follows illustrates using closures to avoid double JavaScript encoding. 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. At a basic level XSS works by tricking your application into inserting a