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%>. 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. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. 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. How common is DOM-based cross-site scripting? This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. 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. 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. The rendered output would now become. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. What's the best way to prevent XSS attacks? | TechTarget Variables should only be placed in a CSS property value. Once you've found where the source is being read, you can use the JavaScript debugger to add a break point and follow how the source's value is used. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. The best way to fix DOM based cross-site scripting is to use the right output method (sink). In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. A Computer Science portal for geeks. Do your applications use this vulnerable package? This difference makes JavaScript encoding a less viable weapon in our fight against XSS. 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. Event handlers such as onload and onerror can be used in conjunction with these elements. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. DOM-Based Cross-Site Scripting. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. If these methods are provided with untrusted input, then an XSS vulnerability could result. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. Misconceptions abound related to the proper encoding that is required. Information on ordering, pricing, and more. Websites may also store data on the server and reflect it elsewhere. The third cross site scripting attack occurs entirely in the browser. However, depending on the tag which innerText is applied, code can be executed. Thankfully, many sinks where variables can be placed are safe. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. When this happens, a script on the web page selects the URL variable and executes the code it contains. 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(). Download the latest version of Burp Suite. In many cases, JavaScript encoding does not stop attacks within an execution context. If your code looked like the following, you would need to only double JavaScript encode input data. 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 is a Safe Sink and will automatically CSS encode data in it. 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). The example that follows illustrates using closures to avoid double JavaScript encoding. 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. React XSS Guide: Examples and Prevention - StackHawk The most common one would be adding it to an href or src attribute of an tag. For example, you might need to close some existing elements before using your JavaScript payload. 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. For instance, jQuery's attr() function can change the attributes of DOM elements. 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. document.CreateTextNode () and append it in the appropriate DOM location. Trusted Types heavily reduce the DOM XSS attack surface of your application. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. Other CSS Contexts are unsafe and you should not place variable data in them. Sometimes you can't change the offending code. How to Prevent DOM-based Cross-site Scripting - blackMORE Ops JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. Catch critical bugs; ship more secure software, more quickly. You might find that the source gets assigned to other variables. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. Please note, element.setAttribute is only safe for a limited number of attributes. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. 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. Free, lightweight web application security scanning for CI/CD. The complication is compounded by the differing meanings and treatment of encoded values within each subcontext (HTML, HTML attribute, URL, and CSS) within the execution context. placed in an HTML Attribute. 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. - owasp-CheatSheetSeries . You need to work through each available source in turn, and test each one individually. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. I will show you three examples of DOM-based XSS attacks in this article. Automatic encoding and escaping functions are built into most frameworks. For example, this is the case if you're loading a third-party library from a CDN. 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). 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. What's the difference between Pro and Enterprise Edition? HTML Context refers to inserting a variable between two basic HTML tags like a