You might find that the source gets assigned to other variables. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. 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. DOM-based XSS is an advanced XSS attack. In other words, add a level of indirection between untrusted input and specified object properties. 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. Document Object Model (DOM) Based XSS. There are a variety of sinks that are relevant to DOM-based vulnerabilities. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Types of XSS (Cross-site Scripting) - Acunetix Fewer XSS bugs appear in applications built with modern web frameworks. If your code looked like the following, you would need to only double JavaScript encode input data. 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. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. The Unicode standard has a list of code charts you can use to find the chart containing your characters. 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. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. This is why you would need to HTML encode too. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. The DOM-based cross-site scripting requires the user to open an infected page. Get your questions answered in the User Forum. 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. What's the best way to prevent XSS attacks? | TechTarget DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks Safe HTML Attributes include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. Preventing XSS in ASP.NET - Code Envato Tuts+ Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. What's the difference between Pro and Enterprise Edition? 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. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. In some . The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. 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. XSS sinks are places where variables are placed into your webpage. The DOM is a programming interface. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. This variable includes some characters which are used in XSS attacks, namely <, " and >. 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. 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 primary difference is where the attack is injected into the application. Ideally, the correct way to apply encoding and avoid the problem stated above is to server-side encode for the output context where data is introduced into the application. WAFs also dont address the root cause of an XSS vulnerability. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. When other users load affected pages the attacker's scripts will run, enabling the attacker to steal cookies and session tokens, change the contents of the web page through DOM manipulation or redirect the browser to another page. I will show you three examples of DOM-based XSS attacks in this article. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. Login here. 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. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. The data is subsequently read from the DOM by the web application and outputted to the browser. This helps quickly identify a large chunk of violations. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. 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. For example, this is the case if you're loading a third-party library from a CDN. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. 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. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. Sometimes users need to author HTML. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. 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. 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. Output encoding here will prevent XSS, but it will break the intended functionality of the application. 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. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. 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%>. 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. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Avoid populating the following methods with untrusted data. No single technique will solve XSS. Trusted Types force you to process a value. 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. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. 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. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Automatic encoding and escaping functions are built into most frameworks. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. //any code passed into lName is now executable. 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. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. It is the process of converting untrusted . Tag helpers will also encode input you use in tag parameters. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Its the same with computer security. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. HTML Context refers to inserting a variable between two basic HTML tags like a