For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. For details, see the Google Developers Site Policies. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. 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. Read about other types of cross-site scripting attacks. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. 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. What's the difference between Pro and Enterprise Edition? Save time/money. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. In a DOM-based attacks, the HTTP response on the server side does not change. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. If your code looked like the following, you would need to only double JavaScript encode input data. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. 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. We want to hear from you! So XSS has already been around for a while. Therefore, the primary recommendation is to avoid including untrusted data in this context. Sometimes you can't change the offending code. There are some further things to consider: Security professionals often talk in terms of sources and sinks. HTML encoding takes characters such as < and changes them into a safe form like < Before putting untrusted data into an HTML attribute ensure it's HTML encoded. 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 ">". The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. Document Object Model (DOM) Based XSS. Encode all characters with the %HH encoding format. 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. WAFs are unreliable and new bypass techniques are being discovered regularly. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. XSS Prevention & Mitigation. //any code passed into lName is now executable. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. At a basic level XSS works by tricking your application into inserting a