Content Security Policy - An allowlist that prevents content being loaded. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. Customization of the safe list only affects encoders sourced via DI. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. This is because these sinks treat the variable as text and will never execute it. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Avoid treating untrusted data as code or markup within JavaScript code. 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. 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. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. This is a Safe Sink and will automatically CSS encode data in it. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. element.SetAttribute () element [attribute]= This cheatsheet is a list of techniques to prevent or limit the impact of XSS. However, you may still find vulnerable code in the wild. - owasp-CheatSheetSeries . Summary. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. The example that follows illustrates using closures to avoid double JavaScript encoding. Here are some examples of encoded values for specific characters. In DOM-based cross-site scripting, the HTML source code and response of the attack . Read more about DOM-based cross-site scripting. XSS sinks are places where variables are placed into your webpage. The enterprise-enabled dynamic web vulnerability scanner. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. Also, XSS attacks always execute in the browser. WAFs are unreliable and new bypass techniques are being discovered regularly. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. Avoid populating the following methods with untrusted data. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Some examples of DOM-based XSS attacks include: 1. Based on this context, you need to refine your input to see how it is processed. For instance, jQuery's attr() function can change the attributes of DOM elements. Thankfully, many sinks where variables can be placed are safe. 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. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. 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. For DOM XSS, the attack is injected into the application during runtime in the client directly. This brings up an interesting design point. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. 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. XSS Prevention & Mitigation. DOM-based XSS: In this type of attack, the attacker injects malicious code into a web page that is executed on the client-side within the Document Object Model (DOM) of the web page. //any code passed into lName is now executable. The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. Read the entire Acunetix Web Application Vulnerability Report. In many cases the context isn't always straightforward to discern. Don't mutate DOM directly. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. Safe list ranges are specified as Unicode code charts, not languages. In the case above, JavaScript encoding does not mitigate against DOM based XSS. For each location where your string appears within the DOM, you need to identify the context. What would be displayed in the input text field would be "Johnson & Johnson". 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. Trusted Types give you the tools to write, security review, and maintain applications free of DOM XSS vulnerabilities by making the dangerous web API functions secure by default. If you need to render different content, use innerText instead of innerHTML. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. 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. There are 3 primary types of cross-site scripting: DOM-based XSS. Trusted Types work by locking down the following risky sink functions. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. DOM-Based Cross-Site Scripting. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. All of this code originates on the server, which means it is the application owner's responsibility to make it safe from XSS, regardless of the type of XSS flaw it is. Before putting untrusted data inside an HTML element ensure it's HTML encoded. At a basic level XSS works by tricking your application into inserting a