Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. Semgrep rule to identify above dom xss link. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". The #redir route is executed by another file, redir.html. Script manipulation: <script src> and setting text content of <script> elements. 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. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. This means you will need to use alternative elements like img or iframe. 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 only safe location for placing variables in JavaScript is inside a quoted data value. The most common one would be adding it to an href or src attribute of an tag. 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). Read about other types of cross-site scripting attacks. 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. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. CSS Contexts refer to variables placed into inline CSS. Then client-side encode (using a JavaScript encoding library such as node-esapi) for the individual subcontext (DOM methods) which untrusted data is passed to. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Use only safe functions like document.innerText and document.textContent. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Already got an account? DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. WAFs also dont address the root cause of an XSS vulnerability. The DOM, or Document Object Model, is the structural format used to . Download the latest version of Burp Suite. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. 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. Therefore, the primary recommendation is to avoid including untrusted data in this context. The rendered output would now become. The name originated from early versions of the attack where stealing data cross-site was the primary focus. 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. A Computer Science portal for geeks. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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. Get started with Burp Suite Enterprise Edition. One of the simplest ways of doing this is to deliver your exploit via an iframe: In this example, the src attribute points to the vulnerable page with an empty hash value. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. If you have to use user input on your page, always use it in the text context, never as HTML tags or any other potential code. You should apply HTML attribute encoding to variables being placed in most HTML attributes. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. Variables should only be placed in a CSS property value. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. If you must, the following examples describe some approaches that do and do not work. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. For that, first create a policy. Despite being rare, they may cause serious problems and only a few scanners can detect them. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). If your code looked like the following, you would need to only double JavaScript encode input data. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. Now only JavaScript encoding on server side. URL Contexts refer to variables placed into a URL. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Each encoder, Html, JavaScript and Url, must be configured separately. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. Validation can be a useful tool in limiting XSS attacks. 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. Information on ordering, pricing, and more. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Its the same with computer security. For example, you might need to close some existing elements before using your JavaScript payload. A rendering context is associated with the parsing of HTML tags and their attributes. Please refer to the list below for details. Get the latest content on web security in your inbox each week. All other contexts are unsafe and you should not place variable data in them. 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. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". Each parser has distinct and separate semantics in the way they can possibly execute script code which make creating consistent rules for mitigating vulnerabilities in various contexts difficult. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. 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(). For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. 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. Stored XSS is considered the most damaging type of XSS attack. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. You might find that the source gets assigned to other variables. This document only discusses JavaScript bugs which lead to XSS. For each location where your string appears within the DOM, you need to identify the context. Sometimes you can't change the offending code. A list of safe HTML attributes is provided in the Safe Sinks section. Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Read the entire Acunetix Web Application Vulnerability Report. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. XSS Prevention & Mitigation. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. . There will be situations where you use a URL in different contexts. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. 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. 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. However, depending on the tag which innerText is applied, code can be executed. 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. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. 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. This is common when you want users to be able to customize the look and feel of their webpages. 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 best way to fix DOM based cross-site scripting is to use the right output method (sink). As we use reCAPTCHA, you need to be able to access Google's servers to use this function. Get the latest content on web security in your inbox each week. In DOM-based cross-site scripting, the HTML source code and response of the attack . Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. There are a variety of sinks that are relevant to DOM-based vulnerabilities. Output Encoding. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. //any code passed into lName is now executable. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. //The following does NOT work because of the encoded "(" and ")". In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. 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. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Document Object Model (DOM) Based XSS. Policies are factories for Trusted Types that enforce certain security rules on their input: This code creates a policy called myEscapePolicy that can produce TrustedHTML objects via its createHTML() function. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. (It's free!). In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. 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. When a browser is rendering HTML and any other associated content like CSS or JavaScript, it identifies various rendering contexts for the different kinds of input and follows different rules for each context. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. In those cases, create a Trusted Type object yourself. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. //The following does NOT work because the event handler is being set to a string. 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. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. 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. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. Want to track your progress and have a more personalized learning experience? Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. It uses HTML attribute encoding rules whenever you use the @ directive. 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. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. For instance, jQuery's attr() function can change the attributes of DOM elements. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. Reduce risk. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. There are 3 primary types of cross-site scripting: DOM-based XSS. 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. We are looking for web developers to participate in user research, product testing, discussion groups and more. 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. Trusted Types require you to process the data before passing it to the above sink functions. Parsing HTML input is difficult, if not impossible. 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. HTML Context refers to inserting a variable between two basic HTML tags like a