Get started with Burp Suite Professional. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. For example, using the default configuration you might use a Razor HtmlHelper like so; When you view the source of the web page you will see it has been rendered as follows, with the Chinese text encoded; To widen the characters treated as safe by the encoder you would insert the following line into the ConfigureServices() method in startup.cs; This example widens the safe list to include the Unicode Range CjkUnifiedIdeographs. 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. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. For that, first create a policy. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Any variable that does not go through this process is a potential weakness. It is an informational message with a simple alert. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. 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. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. Validation becomes more complicated when accepting HTML in user input. What's the difference between Pro and Enterprise Edition? For example, when your application passes a string to innerHTML, the browser sends the following report: This says that in https://my.url.example/script.js on line 39 innerHTML was called with the string beginning with <img src=x. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. If this is the case, you'll need to use the search function again to track these variables and see if they're passed to a sink. More info about Internet Explorer and Microsoft Edge. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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). This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. The line above could have possibly worked to render a link. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. Use a trusted and verified library to escape HTML inputs. DOM XSS stands for Document Object Model-based Cross-site Scripting. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. Variables should only be placed in a CSS property value. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. document.CreateTextNode () and append it in the appropriate DOM location. Output Encoding and HTML Sanitization help address those gaps. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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. The following are some of the main sinks that can lead to DOM-XSS vulnerabilities: The following jQuery functions are also sinks that can lead to DOM-XSS vulnerabilities: In addition to the general measures described on the DOM-based vulnerabilities page, you should avoid allowing data from any untrusted source to be dynamically written to the HTML document. The best way to fix DOM based cross-site scripting is to use the right output method (sink). \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Avoid treating untrusted data as code or markup within JavaScript code. Get your questions answered in the User Forum. As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. Use only safe functions like document.innerText and document.textContent. In those cases, create a Trusted Type object yourself. 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. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. These methods constitute the HTML Subcontext within the Execution Context. For instance, jQuery's attr() function can change the attributes of DOM elements. Some pure DOM-based vulnerabilities are self-contained within a single page. Trusted Types force you to process a value somehow, but don't yet define what the exact processing rules are, and whether they are safe. your framework), you should be able to mitigate all XSS vulnerabilities. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. Save time/money. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. This is a Safe Sink and will automatically URL encode data in it. Download the latest version of Burp Suite. If you're using JavaScript to change a CSS property, look into using style.property = x. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. 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. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. There are a couple of options for fixing a Trusted Type violation. The reasoning behind this is to protect against unknown or future browser bugs (previous browser bugs have tripped up parsing based on the processing of non-English characters). Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). The reason why you only need to double JavaScript encode is that the customFunction function did not itself pass the input to another method which implicitly or explicitly called eval If firstName was passed to another JavaScript method which implicitly or explicitly called eval() then <%=doubleJavaScriptEncodedData%> above would need to be changed to <%=tripleJavaScriptEncodedData%>. 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. This variable includes some characters which are used in XSS attacks, namely <, " and >. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). // is an example of untrusted data that was properly JavaScript encoded but still executes. However the opposite is the case with HTML encoding. Learn more about types of cross-site scripting attacks Script manipulation: <script src> and setting text content of <script> elements. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). 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 . Its the same with computer security. 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. In a DOM-based attacks, the HTTP response on the server side does not change. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. This can be done via a function such as: There are several methods and attributes which can be used to directly render HTML content within JavaScript. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document.

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