dom based cross site scripting prevention

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. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. These methods constitute the HTML Subcontext within the Execution Context. 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 is a Safe Sink and will automatically CSS encode data in it. Trusted Types heavily reduce the DOM XSS attack surface of your application. RULE #1 - HTML Escape then JavaScript Escape Before Inserting Untrusted Data into HTML Subcontext within the Execution Context, RULE #2 - JavaScript Escape Before Inserting Untrusted Data into HTML Attribute Subcontext within the Execution Context, RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context, RULE #4 - JavaScript Escape Before Inserting Untrusted Data into the CSS Attribute Subcontext within the Execution Context, RULE #5 - URL Escape then JavaScript Escape Before Inserting Untrusted Data into URL Attribute Subcontext within the Execution Context, RULE #6 - Populate the DOM using safe JavaScript functions or properties, RULE #7 - Fixing DOM Cross-site Scripting Vulnerabilities, Guidelines for Developing Secure Applications Utilizing JavaScript, GUIDELINE #1 - Untrusted data should only be treated as displayable text, GUIDELINE #2 - Always JavaScript encode and delimit untrusted data as quoted strings when entering the application when building templated JavaScript, GUIDELINE #3 - Use document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar to build dynamic interfaces, GUIDELINE #4 - Avoid sending untrusted data into HTML rendering methods, GUIDELINE #5 - Avoid the numerous methods which implicitly eval() data passed to it, Utilizing an Enclosure (as suggested by Gaz), GUIDELINE #6 - Use untrusted data on only the right side of an expression, GUIDELINE #7 - When URL encoding in DOM be aware of character set issues, GUIDELINE #8 - Limit access to object properties when using object[x] accessors, GUIDELINE #9 - Run your JavaScript in a ECMAScript 5 canopy or sandbox, GUIDELINE #10 - Don't eval() JSON to convert it to native JavaScript objects, Common Problems Associated with Mitigating DOM Based XSS, Insecure Direct Object Reference Prevention, Creative Commons Attribution 3.0 Unported License. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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. 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. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. 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. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Never rely on validation alone. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. DOMPurify supports Trusted Types and will return sanitized HTML wrapped in a TrustedHTML object such that the browser does not generate a violation.CautionIf the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. CSS Contexts refer to variables placed into inline CSS. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. 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. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Variables should not be interpreted as code instead of text. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. If you need to render different content, use innerText instead of innerHTML. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. 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. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. Websites may also store data on the server and reflect it elsewhere. If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. 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. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. placed in an HTML Attribute. The following is an example vulnerability which occurs in the JavaScript context and HTML subcontext: Let's look at the individual subcontexts of the execution context in turn. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. . 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). 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. Misconceptions abound related to the proper encoding that is required. If you directly access an encoder via System.Text.Encodings.Web. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. This document only discusses JavaScript bugs which lead to XSS. How to prevent DOM-based cross-site scripting? One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Reduce risk. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. The world's #1 web penetration testing toolkit. It will not always prevent XSS. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. For each potential source, such as location, you first need to find cases within the page's JavaScript code where the source is being referenced. Please refer to the list below for details. For DOM XSS, the attack is injected into the application during runtime in the client directly. It's important to remember that some of these are also potential sources and sinks for DOM XSS. It uses HTML attribute encoding rules whenever you use the @ directive. If you must, the following examples describe some approaches that do and do not work. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. OWASP recommends these in all circumstances. You might find that the source gets assigned to other variables. 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. 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. 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. 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. 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. 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. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. In a DOM-based attacks, the HTTP response on the server side does not change. 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 only safe location for placing variables in JavaScript is inside a quoted data value. Content Security Policy - An allowlist that prevents content being loaded. \u0064\u006f\u0063\u0075\u006d\u0065\u006e\u0074, \u0077\u0072\u0069\u0074\u0065\u006c\u006e, "\u0048\u0065\u006c\u006c\u006f\u0020\u0057\u006f\u0072\u006c\u0064", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0031\u0029", "url(<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(companyName))%>)", '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(userRelativePath))%>', "<%= Encode.forJavaScript(untrustedData) %>", "<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>", "customFunction('<%=doubleJavaScriptEncodedData%>', y)", //HTML encoding is happening in JavaScript, "javascript:myFunction('<%=untrustedData%>', 'test');", "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(untrustedData)) %>', 'test');",