The document.write sink works with script elements, so you can use a simple payload, such as the one below: Note, however, that in some situations the content that is written to document.write includes some surrounding context that you need to take account of in your exploit. XSS is one of the most common and dangerous web vulnerabilities, and it is . In many cases, JavaScript encoding does not stop attacks within an execution context. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. In the case above, JavaScript encoding does not mitigate against DOM based XSS. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. For example, when your application passes a string to innerHTML, the browser sends the following report: This says that in https://my.url.example/script.js on line 39 innerHTML was called with the string beginning with <img src=x. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. WSTG - v4.1 | OWASP Foundation Its the same with computer security. It uses HTML attribute encoding rules whenever you use the @ directive. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. Do your applications use this vulnerable package? React XSS Guide: Examples and Prevention - StackHawk 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. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. For each location where your string appears within the DOM, you need to identify the context. Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. 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). Validation can be a useful tool in limiting XSS attacks. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. However the opposite is the case with HTML encoding. In a DOM-based attacks, the HTTP response on the server side does not change. So XSS has already been around for a while. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. Developers should use the following prevention steps to avoid introducing XSS into their application. The good news is that if user input is handled properly at the foundation level (e.g. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. (It's free!). //The following does NOT work because the event handler is being set to a string. As with all other Cross-site Scripting (XSS) vulnerabilities, this type of attack also relies on insecure handling of user input on an HTML page. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. By default encoders use a safe list limited to the Basic Latin Unicode range and encode all characters outside of that range as their character code equivalents. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. Cross Site Scripting Prevention Cheat Sheet - OWASP Trusted Types heavily reduce the DOM XSS attack surface of your application. Reduce risk. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". Here are some examples of encoded values for specific characters. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. Canonicalize input, URL Validation, Safe URL verification, Allow-list http and HTTPS URLs only (Avoid the JavaScript Protocol to Open a new Window), Attribute encoder. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. It will not always prevent XSS. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Some examples of DOM-based XSS attacks include: 1. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. Avoid populating the following methods with untrusted data. Browsers change functionality and bypasses are being discovered regularly. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. 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. 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. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. 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. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. See how our software enables the world to secure the web. A list of output encoding libraries is included in the appendix. 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. There are a variety of sinks that are relevant to DOM-based vulnerabilities. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. Reduce the DOM XSS attack surface of your application. 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. When this happens, a script on the web page selects the URL variable and executes the code it contains. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Thankfully, many sinks where variables can be placed are safe. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. What is DOM-based cross-site scripting? - PortSwigger There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. . The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Variables should not be interpreted as code instead of text. The most common one would be adding it to an href or src attribute of an tag. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. What's the best way to prevent XSS attacks? | TechTarget This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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. jQuery used to be extremely popular, and a classic DOM XSS vulnerability was caused by websites using this selector in conjunction with the location.hash source for animations or auto-scrolling to a particular element on the page. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. To detect the possibility of a DOM XSS, you must simulate the attack from the client-side in the users browser using a web application scanner like Acunetix (with DOM-based XSS scanner functionality). Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. What is Cross-Site Scripting (XSS) and How to Prevent It? You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. It is an informational message with a simple alert. Get started with Burp Suite Enterprise Edition. See what Acunetix Premium can do for you. The example that follows illustrates using closures to avoid double JavaScript encoding. Understanding the XSS Threat: A Comprehensive Guide to DOM Based Cross Parsing HTML input is difficult, if not impossible. Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. This means, that no data will be available in server logs. 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. Sometimes you can't change the offending code. The DOM, or Document Object Model, is the structural format used to . What's the difference between Pro and Enterprise Edition? Please refer to the list below for details. 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. You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. 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. Any variable that does not go through this process is a potential weakness. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. 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()). 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. However, you may still find vulnerable code in the wild. The best way to fix DOM based cross-site scripting is to use the right output method (sink). It's important to remember that some of these are also potential sources and sinks for DOM XSS. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks In these cases, HTML Sanitization should be used. In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. At a basic level XSS works by tricking your application into inserting a