If you sanitize content and then modify it afterwards, you can easily void your security efforts. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Trusted Types force you to process a value somehow, but don't yet define what the exact processing rules are, and whether they are safe. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. You should apply HTML attribute encoding to variables being placed in most HTML attributes. If your code looked like the following, you would need to only double JavaScript encode input data. CSS is surprisingly powerful and has been used for many types of attacks. Normally executing JavaScript from a CSS context required either passing javascript:attackCode() to the CSS url() method or invoking the CSS expression() method passing JavaScript code to be directly executed. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. This brings up an interesting design point. The reason why you only need to double JavaScript encode is that the customFunction function did not itself pass the input to another method which implicitly or explicitly called eval If firstName was passed to another JavaScript method which implicitly or explicitly called eval() then <%=doubleJavaScriptEncodedData%> above would need to be changed to <%=tripleJavaScriptEncodedData%>. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. 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. Fewer XSS bugs appear in applications built with modern web frameworks. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. You need to work through each available source in turn, and test each one individually. Working example (no HTML encoding): Normally encoded example (Does Not Work DNW): HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW): If HTML encoding followed the same semantics as JavaScript encoding. These types of attacks typically occur as a result . Get the latest content on web security in your inbox each week. For example, using the default configuration you might use a Razor HtmlHelper like so; When you view the source of the web page you will see it has been rendered as follows, with the Chinese text encoded; To widen the characters treated as safe by the encoder you would insert the following line into the ConfigureServices() method in startup.cs; This example widens the safe list to include the Unicode Range CjkUnifiedIdeographs. Read about other types of cross-site scripting attacks. There are 3 primary types of cross-site scripting: DOM-based XSS. Finally, to fix the problem in our initial code, instead of trying to encode the output correctly which is a hassle and can easily go wrong we would simply use element.textContent to write it in a content like this: It does the same thing but this time it is not vulnerable to DOM based cross-site scripting vulnerabilities. The third cross site scripting attack occurs entirely in the browser. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. The only safe location for placing variables in JavaScript is inside a quoted data value. The example that follows illustrates using closures to avoid double JavaScript encoding. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. DOM Based Attacks. If this isn't possible, then ensure the data is JavaScript encoded. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. 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. For example if you want to use user input to write in a div tag element don't use innerHtml, instead use innerText or textContent. This could lead to an attack being added to a webpage.. for example. Document Object Model (DOM) Based XSS. This is because these sinks treat the variable as text and will never execute it. This helps quickly identify a large chunk of violations. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. The data is subsequently read from the DOM by the web application and outputted to the browser. Trusted Types give you the tools to write, security review, and maintain applications free of DOM XSS vulnerabilities by making the dangerous web API functions secure by default. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. The most common one would be adding it to an href or src attribute of an tag. Safe list ranges are specified as Unicode code charts, not languages. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. For a comprehensive list, check out the DOMPurify allowlist. Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Use only safe functions like document.innerText and document.textContent. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. element.SetAttribute () element [attribute]= OWASP recommends these in all circumstances. Trusted Types require you to process the data before passing it to the above sink functions. 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=. //The following does NOT work because of the encoded ";". DOM-based XSS Examples. If that isn't enough to keep in mind, you have to remember that encodings are lost when you retrieve them using the value attribute of a DOM element. Learn more about types of cross-site scripting attacks This should never be used in combination with untrusted input as this will expose an XSS vulnerability. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. If you need to render different content, use innerText instead of innerHTML. In some . Accelerate penetration testing - find more bugs, more quickly. It will not always prevent XSS. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Save time/money. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Based on this context, you need to refine your input to see how it is processed. This is a Safe Sink and will automatically CSS encode data in it. *Encoder.Default then the default, Basic Latin only safelist will be used. Output Encoding and HTML Sanitization help address those gaps. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. XSS is one of the most common and dangerous web vulnerabilities, and it is . A list of safe HTML attributes is provided in the Safe Sinks section. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Automatic encoding and escaping functions are built into most frameworks. - owasp-CheatSheetSeries . There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Read the entire Acunetix Web Application Vulnerability Report. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Encoding libraries often have a EncodeForJavaScript or similar to support this function. DOM-based cross-site scripting attack DOM-based XSS is also sometimes called "type-0 XSS." It occurs when the XSS vector executes as a result of a DOM modification on a website in a user's browser. Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. There are numerous methods which implicitly eval() data passed to it that must be avoided. 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. One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. The best manual tools to start web security testing. This document only discusses JavaScript bugs which lead to XSS. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. There are a couple of options for fixing a Trusted Type violation. Please note, it is always dangerous design to put untrusted data directly into a command execution context. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. Encode all characters with the %HH encoding format. //any code passed into lName is now executable. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. //The following DOES WORK because the encoded value is a valid variable name or function reference. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. Thankfully, many sinks where variables can be placed are safe. 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. At a basic level XSS works by tricking your application into inserting a