Betty T Lee State Controller Disbursements Bureau, Articles D

This cheatsheet is a list of techniques to prevent or limit the impact of XSS. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. The example that follows illustrates using closures to avoid double JavaScript encoding. Already got an account? Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Using untrusted user data on the left side of the expression allows an attacker to subvert internal and external attributes of the window object, whereas using user input on the right side of the expression doesn't allow direct manipulation. 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. If you directly access an encoder via System.Text.Encodings.Web. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. Don't mutate DOM directly. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Scale dynamic scanning. Use a trusted and verified library to escape HTML inputs. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! The good news is that if user input is handled properly at the foundation level (e.g. It will not always prevent XSS. placed in an HTML Attribute. Document Object Model (DOM) Based XSS. A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. 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. 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. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. All other contexts are unsafe and you should not place variable data in them. Enhance security monitoring to comply with confidence. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. The DOM, or Document Object Model, is the structural format used to . 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. 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. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Sometimes users need to author HTML. The web application dynamically generates a web page that contains this untrusted data. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. 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. 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. DOM-based XSS: DOM-based XSS occurs when an . There are 3 primary types of cross-site scripting: DOM-based XSS. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Output encoding is the primary defense against cross-site scripting vulnerabilities. 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. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Variables should not be interpreted as code instead of text. Ideally, the correct way to apply encoding and avoid the problem stated above is to server-side encode for the output context where data is introduced into the application. Sometimes you can't change the offending code. (It's free!). At a basic level XSS works by tricking your application into inserting a