XSS sinks are places where variables are placed into your webpage. Accelerate penetration testing - find more bugs, more quickly. 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. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. How To Prevent DOM-based Cross-site Scripting | Acunetix 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. 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. The enterprise-enabled dynamic web vulnerability scanner. The DOM is a programming interface. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. The best way to fix DOM based cross-site scripting is to use the right output method (sink). This could lead to an attack being added to a webpage.. for example. 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()). This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. In many cases, JavaScript encoding does not stop attacks within an execution context. Web Application Firewalls - These look for known attack strings and block them. If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. Get started with Burp Suite Professional. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. Encode all characters using the \xHH format. The data is subsequently read from the DOM by the web application and outputted to the browser. For instance, jQuery's attr() function can change the attributes of DOM elements. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. DOM-based XSS is a kind of XSS occurring entirely on the client-side. Each encoder, Html, JavaScript and Url, must be configured separately. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. This is why you would need to HTML encode too. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. 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 . For details, see the Google Developers Site Policies. 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. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. The DOM-based cross-site scripting requires the user to open an infected page. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. One example of an attribute which is thought to be safe is innerText. 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. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks DOM based XSS vulnerabilities therefore have to be prevented on the client side. In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). Content Security Policy - An allowlist that prevents content being loaded. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. However, you may still find vulnerable code in the wild. 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. Some examples of DOM-based XSS attacks include: 1. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. What is XSS? Impact, Types, and Prevention - Bright Security A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). These locations are known as dangerous contexts. HTML Context refers to inserting a variable between two basic HTML tags like a