This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. XSS is one of the most common and dangerous web vulnerabilities, and it is . You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Its the same with computer security. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. Encoding libraries often have a EncodeForJavaScript or similar to support this function. 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. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Read about other types of cross-site scripting attacks. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). For example.. An attacker could modify data that is rendered as $varUnsafe. The primary rule that you must follow to prevent DOM XSS is: sanitize all untrusted data, even if it is only used in client-side scripts. That said, developers need to be aware of problems that can occur when using frameworks insecurely such as: Understand how your framework prevents XSS and where it has gaps. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Each encoder, Html, JavaScript and Url, must be configured separately. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. Reduce the DOM XSS attack surface of your application. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. We will look at eval, href and dangerouslySetHTML vulnerabilities. URL Contexts refer to variables placed into a URL. 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). Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. Validation can be a useful tool in limiting XSS attacks. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. Information on ordering, pricing, and more. 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. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. 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. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. 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. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. The best way to fix DOM based cross-site scripting is to use the right output method (sink). It is an informational message with a simple alert. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. So XSS has already been around for a while. 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. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Record your progression from Apprentice to Expert. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. 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 . Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Please insert your password to refresh your session. 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. Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. When this happens, a script on the web page selects the URL variable and executes the code it contains. In other words, add a level of indirection between untrusted input and specified object properties. 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 world's #1 web penetration testing toolkit. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. If A is double JavaScript encoded then the following if check will return false. These locations are known as dangerous contexts. If you sanitize content and then modify it afterwards, you can easily void your security efforts. Fewer XSS bugs appear in applications built with modern web frameworks. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. Don't mutate DOM directly. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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. Based on this context, you need to refine your input to see how it is processed. 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. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. For example, Acunetix. 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. This is a Safe Sink and will automatically URL encode data in it. Use only safe functions like document.innerText and document.textContent. This helps quickly identify a large chunk of violations. The rendered output would now become. Please note, element.setAttribute is only safe for a limited number of attributes. //any code passed into lName is now executable. There are 3 primary types of cross-site scripting: DOM-based XSS. This is why you would need to HTML encode too. 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. DOM XSS stands for Document Object Model-based Cross-site Scripting. Testing JavaScript execution sinks for DOM-based XSS is a little harder. In those cases, create a Trusted Type object yourself. These types of attacks typically occur as a result . \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. Any variable that does not go through this process is a potential weakness. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. The only safe location for placing variables in JavaScript is inside a quoted data value. Tag helpers will also encode input you use in tag parameters. Trusted Types force you to process a value. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Never rely on validation alone. A Computer Science portal for geeks. The HTML encoded value above is still executable. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. For example, this is the case if you're loading a third-party library from a CDN. Catch critical bugs; ship more secure software, more quickly. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. This means, that no data will be available in server logs. Trusted Types require you to process the data before passing it to the above sink functions. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. Use a CSP as an additional layer of defense and have a look at the. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. This variable includes some characters which are used in XSS attacks, namely <, " and >. 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. The most common one would be adding it to an href or src attribute of an tag. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. It uses HTML attribute encoding rules whenever you use the @ directive. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. DOM-based XSS is an advanced XSS attack. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. //The following does NOT work because the event handler is being set to a string. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. One example of an attribute which is thought to be safe is innerText. 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 primary difference is where the attack is injected into the application. This cheat sheet provides guidance to prevent XSS vulnerabilities. Reduce risk. Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. Encode all characters with the %HH encoding format. If you pollute a river, it'll flow downstream somewhere. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. If you need to render different content, use innerText instead of innerHTML. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. Get started with Burp Suite Professional. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. Semgrep rule to identify above dom xss link. In DOM-based cross-site scripting, the HTML source code and response of the attack . The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. 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. 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. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. For example, websites often reflect URL parameters in the HTML response from the server. DOM-based XSS Examples. DOM-based XSS is a kind of XSS occurring entirely on the client-side. It will not always prevent XSS. Read more about DOM-based cross-site scripting. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. 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. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. 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. What's the difference between Pro and Enterprise Edition? CSS Contexts refer to variables placed into inline CSS. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. Event handlers such as onload and onerror can be used in conjunction with these elements. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Accelerate penetration testing - find more bugs, more quickly. DOM based XSS vulnerabilities therefore have to be prevented on the client side. Safe list ranges are specified as Unicode code charts, not languages. Input validation. For details, see the Google Developers Site Policies. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. In JavaScript code, the main context is JavaScript but with the right tags and context closing characters, an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. 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. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Want to track your progress and have a more personalized learning experience? The DOM is a programming interface. Scale dynamic scanning. This can be done via a function such as: A list of output encoding libraries is included in the appendix. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. eval An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. For example. It is also impossible to protect against such client-side attacks using WAFs. If you must, the following examples describe some approaches that do and do not work. The name originated from early versions of the attack where stealing data cross-site was the primary focus. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. XSS Prevention & Mitigation. Some pure DOM-based vulnerabilities are self-contained within a single page. This is where Output Encoding and HTML Sanitization are critical. 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. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. All other contexts are unsafe and you should not place variable data in them. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Level up your hacking and earn more bug bounties. 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. DOM-based XSS simply means a cross-site scripting vulnerability that occurs in the DOM ( Document Object Model) of your site rather than in HTML. The Unicode standard has a list of code charts you can use to find the chart containing your characters. 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 innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. DOM-based cross-site scripting is the de-facto name for XSS bugs that are the result of active browser-side content on a page, typically JavaScript, obtaining user input and then doing something unsafe with it, leading to the execution of injected code. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. The good news is that if user input is handled properly at the foundation level (e.g. Read the entire Acunetix Web Application Vulnerability Report. On the client side, the HTTP response does not change but the script executes in malicious manner. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. 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. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). Output Encoding. If you're using JavaScript for writing to HTML, look at the .textContent attribute as it is a Safe Sink and will automatically HTML Entity Encode. A script within the later response contains a sink which then processes the data in an unsafe way. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. 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. The enterprise-enabled dynamic web vulnerability scanner. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. The name originated from early versions of the attack where stealing data cross-site was the primary focus. As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. The DOM-based cross-site scripting requires the user to open an infected page. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. At a basic level XSS works by tricking your application into inserting a