When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). Therefore, the primary recommendation is to avoid including untrusted data in this context. How to prevent DOM-based cross-site scripting? 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. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. In the case above, JavaScript encoding does not mitigate against DOM based XSS. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. These locations are known as dangerous contexts. However, if the pages returned from your web application utilize a content type of text/xhtml or the file type extension of *.xhtml then HTML encoding may not work to mitigate against XSS. Get started with Burp Suite Enterprise Edition. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Developers should use the following prevention steps to avoid introducing XSS into their application. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Enhance security monitoring to comply with confidence. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. The example that follows illustrates using closures to avoid double JavaScript encoding. 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. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. For example, you might need to close some existing elements before using your JavaScript payload. 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. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. 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. 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. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. eval Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. 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. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. Read the entire Acunetix Web Application Vulnerability Report. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. A list of safe HTML attributes is provided in the Safe Sinks section. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. This means, that no data will be available in server logs. The best manual tools to start web security testing. The data is subsequently read from the DOM by the web application and outputted to the browser. 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%>. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. Fewer XSS bugs appear in applications built with modern web frameworks. WAFs are unreliable and new bypass techniques are being discovered regularly. 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. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Sometimes you can't change the offending code. For DOM XSS, the attack is injected into the application during runtime in the client directly. 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. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Login here. How to detect DOM-based cross-site scripting? Each variable used in the user interface should be passed through an output encoding function. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. These types of attacks typically occur as a result . Some examples of DOM-based XSS attacks include: 1. Otherwise, again, your security efforts are void. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The DOM-based cross-site scripting requires the user to open an infected page. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). Rather, a malicious change in the DOM environment causes client code to run unexpectedly. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. Please note, it is always dangerous design to put untrusted data directly into a command execution context. If your code looked like the following, you would need to only double JavaScript encode input data. Output Encoding and HTML Sanitization help address those gaps. If this isn't possible, then ensure the data is JavaScript encoded. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. 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. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Safe list ranges are specified as Unicode code charts, not languages. For example, if your string appears within a double-quoted attribute then try to inject double quotes in your string to see if you can break out of the attribute. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. The DOM is a programming interface. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. It is a simple yet effective way to harvest passwords using only the victims browser. How common is DOM-based cross-site scripting? DOM based XSS vulnerabilities therefore have to be prevented on the client side. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. This is why you would need to HTML encode too. Get the latest content on web security in your inbox each week. Output encoding is the primary defense against cross-site scripting vulnerabilities. DOM-based XSS is an advanced XSS attack. (It's free!). The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. DOM-based cross-site scripting is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. URL Contexts refer to variables placed into a URL. //any code passed into lName is now executable. Acunetix developers and tech agents regularly contribute to the blog. Cross-Site Scripting (XSS) is a misnomer. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. If you sanitize content and then modify it afterwards, you can easily void your security efforts. If A is double JavaScript encoded then the following if check will return false. Misconceptions abound related to the proper encoding that is required. 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. 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. 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. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. All other contexts are unsafe and you should not place variable data in them. 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. 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. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. When this happens, a script on the web page selects the URL variable and executes the code it contains. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). For a comprehensive list, check out the DOMPurify allowlist. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. 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. In many cases, JavaScript encoding does not stop attacks within an execution context. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . 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. OWASP recommends DOMPurify for HTML Sanitization. Before putting untrusted data inside an HTML element ensure it's HTML encoded. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. 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. your framework), you should be able to mitigate all XSS vulnerabilities. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. For example: To make dynamic updates to HTML in the DOM safe, we recommend: The HTML attribute subcontext within the execution context is divergent from the standard encoding rules. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. More info about Internet Explorer and Microsoft Edge. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. //The following does NOT work because of the encoded "(" and ")". Output encoding here will prevent XSS, but it will break the intended functionality of the application. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. Avoid treating untrusted data as code or markup within JavaScript code. . However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. You should apply HTML attribute encoding to variables being placed in most HTML attributes. Validate all data that flows into your application from the server or a third-party API. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. 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).