The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. 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 attack functions by manipulating the internal model of the webpage within the browser known as the DOM and are referred to as DOM based attacks . 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). //The following does NOT work because of the encoded "(" and ")". For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. For example, this is the case if you're loading a third-party library from a CDN. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. In many cases the context isn't always straightforward to discern. 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. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Practise exploiting vulnerabilities on realistic targets. . For example. 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). With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. 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. Get the latest content on web security in your inbox each week. The Unicode standard has a list of code charts you can use to find the chart containing your characters. There will be situations where you use a URL in different contexts. 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. For example, Acunetix. 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. Already got an account? Testing JavaScript execution sinks for DOM-based XSS is a little harder. Its the same with computer security. Get started with Burp Suite Enterprise Edition. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. A script within the later response contains a sink which then processes the data in an unsafe way. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. The HTML encoded value above is still executable. What would be displayed in the input text field would be "Johnson & Johnson". This variable includes some characters which are used in XSS attacks, namely <, " and >. 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. The problem is that if companyName had the value "Johnson & Johnson". You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. 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. Variables should not be interpreted as code instead of text. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. The most common one would be adding it to an href or src attribute of an
tag. So XSS has already been around for a while. How to prevent DOM-based cross-site scripting? element.SetAttribute () element [attribute]= For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. This can be done via a function such as: When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. WAFs also dont address the root cause of an XSS vulnerability. If you're using JavaScript to change a CSS property, look into using style.property = x. Some pure DOM-based vulnerabilities are self-contained within a single page. 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. There are a couple of options for fixing a Trusted Type violation. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Output encoding is not perfect. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. DOM Based Attacks. At a basic level XSS works by tricking your application into inserting a