dom based cross site scripting prevention

There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. 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. 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. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. The example that follows illustrates using closures to avoid double JavaScript encoding. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. All the Acunetix developers come with years of experience in the web security sphere. 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. This is why you would need to HTML encode too. For a comprehensive list, check out the DOMPurify allowlist. //any code passed into lName is now executable. Each variable in a web application needs to be protected. Now only JavaScript encoding on server side. . The #redir route is executed by another file, redir.html. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. The world's #1 web penetration testing toolkit. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. Read the entire Acunetix Web Application Vulnerability Report. . With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Validation becomes more complicated when accepting HTML in user input. Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. Login here. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. DOM-based XSS is an advanced XSS attack. Common injection vectors include document.url, document.location, and document.referrer objects. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Limit access to object properties when using object[x] accessors (Mike Samuel). However, depending on the tag which innerText is applied, code can be executed. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. URL Contexts refer to variables placed into a URL. A Computer Science portal for geeks. 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. 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. 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. OWASP recommends these in all circumstances. Note that browsers behave differently with regards to URL-encoding, Chrome, Firefox, and Safari will URL-encode location.search and location.hash, while IE11 and Microsoft Edge (pre-Chromium) will not URL-encode these sources. So XSS has already been around for a while. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Output encoding is the primary defense against cross-site scripting vulnerabilities. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. For example, Acunetix. There are two ways to do this. The primary difference is where the attack is injected into the application. Get your questions answered in the User Forum. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Customization of the safe list only affects encoders sourced via DI. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. //The following does NOT work because the event handler is being set to a string. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. 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. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. 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. See what Acunetix Premium can do for you. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Encode all characters with the %HH encoding format. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. Avoid populating the following methods with untrusted data. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". There are several methods and attributes which can be used to directly render HTML content within JavaScript. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. 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. If your code looked like the following, you would need to only double JavaScript encode input data. Acunetix developers and tech agents regularly contribute to the blog. This means you will need to use alternative elements like img or iframe. Learn the details here including XSS prevention methods. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. 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. 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. 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. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. 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. Use a trusted and verified library to escape HTML inputs. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. By default encoders use a safe list limited to the Basic Latin Unicode range and encode all characters outside of that range as their character code equivalents. 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. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. However the opposite is the case with HTML encoding. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. Save time/money. Each encoder, Html, JavaScript and Url, must be configured separately. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. You might find that the source gets assigned to other variables. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. DOM-based XSS attacks seek to exploit the DOM in a simple two step process: Create a Source: Inject a malicious script into a property found to be suceptible to DOM-based XSS attacks. Encoding libraries often have a EncodeForJavaScript or similar to support this function. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. Please insert your password to refresh your session. 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. At a basic level XSS works by tricking your application into inserting a