Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. If you're using JavaScript for writing to HTML, look at the .textContent attribute as it is a Safe Sink and will automatically HTML Entity Encode. 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). An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Customization of the safe list only affects encoders sourced via DI. 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. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. How to prevent DOM-based cross-site scripting? See Browser compatibility for up-to-date cross-browser support information.Key TermDOM-based cross-site scripting happens when data from a user controlled source (like user name, or redirect URL taken from the URL fragment) reaches a sink, which is a function like eval() or a property setter like .innerHTML, that can execute arbitrary JavaScript code. 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. The primary rule that you must follow to prevent DOM XSS is: sanitize all untrusted data, even if it is only used in client-side scripts. 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. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. 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. Always pass untrusted input as a query string value. Scale dynamic scanning. Free, lightweight web application security scanning for CI/CD. You might find that the source gets assigned to other variables. WAFs also dont address the root cause of an XSS vulnerability. The good news is that if user input is handled properly at the foundation level (e.g. 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. 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. 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. 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. When this happens, a script on the web page selects the URL variable and executes the code it contains. Using the right combination of defensive techniques is necessary to prevent XSS. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. For example, Acunetix. //The following DOES WORK because the encoded value is a valid variable name or function reference. URL Contexts refer to variables placed into a URL. 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. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! There are a variety of sinks that are relevant to DOM-based vulnerabilities. If this isn't possible, then ensure the data is JavaScript encoded. You need to work through each available source in turn, and test each one individually. Understanding the XSS Threat: A Comprehensive Guide to DOM Based Cross In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. This is common when you want users to be able to customize the look and feel of their webpages. For a comprehensive list, check out the DOMPurify allowlist. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. DOM based Cross Site Scripting - Client-Side Attacks on Browsers - SCIP Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. 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. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. 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. It is the process of converting untrusted . 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. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Sometimes you can't change the offending code. These locations are known as dangerous contexts. Encode all characters with the %HH encoding format. View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. In some . Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. A Computer Science portal for geeks. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. DOM XSS stands for Document Object Model-based Cross-site Scripting. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. Its critical to use quotation marks like " or ' to surround your variables. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. For details, see the Google Developers Site Policies. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. Validation can be a useful tool in limiting XSS attacks. Its the same with computer security. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Read more about DOM-based cross-site scripting. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. OWASP recommends these in all circumstances. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). The third cross site scripting attack occurs entirely in the browser. Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. The most common one would be adding it to an href or src attribute of an tag. WAFs are unreliable and new bypass techniques are being discovered regularly. For example. DOM-based XSS Examples. This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against XSS. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. The data is subsequently read from the DOM by the web application and outputted to the browser. You should apply HTML attribute encoding to variables being placed in most HTML attributes. At a basic level XSS works by tricking your application into inserting a