We are looking for web developers to participate in user research, product testing, discussion groups and more. How to detect DOM-based cross-site scripting? 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. 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. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. 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. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Perhaps the non-conforming functionality is not needed anymore or can be rewritten in a modern way without using the error-prone functions?Don'tel.innerHTML = '<img src=xyz.jpg>'; Doel.textContent = '';const img = document.createElement('img');img.src = 'xyz.jpg';el.appendChild(img); Some libraries already generate Trusted Types that you can pass to the sink functions. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Websites may also store data on the server and reflect it elsewhere. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. A rendering context is associated with the parsing of HTML tags and their attributes. In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template. Free, lightweight web application security scanning for CI/CD. Therefore, the primary recommendation is to avoid including untrusted data in this context. There are numerous methods which implicitly eval() data passed to it that must be avoided. 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. I will show you three examples of DOM-based XSS attacks in this article. Use only safe functions like document.innerText and document.textContent. These methods constitute the HTML Subcontext within the Execution Context. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. In the case above, JavaScript encoding does not mitigate against DOM based XSS. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. Examples of safe attributes includes: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. When this happens, a script on the web page selects the URL variable and executes the code it contains. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). This means, that no data will be available in server logs. This variable includes some characters which are used in XSS attacks, namely <, " and >. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Its the same with computer security. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Now only JavaScript encoding on server side. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. 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. Trusted Types require you to process the data before passing it to the above sink functions. your framework), you should be able to mitigate all XSS vulnerabilities. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. 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. . 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. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Don't use untrusted input as part of a URL path. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. The example that follows illustrates using closures to avoid double JavaScript encoding. Variables should not be interpreted as code instead of text. Script manipulation: <script src> and setting text content of <script> elements. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). Safe HTML Attributes include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. CSS Contexts refer to variables placed into inline CSS. 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. This is a Safe Sink and will automatically URL encode data in it. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. 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. We want to hear from you! Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. Sometimes users need to author HTML. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. It will not always prevent XSS. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Summary. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. In these cases, HTML Sanitization should be used. 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. This behavior was often implemented using a vulnerable hashchange event handler, similar to the following: As the hash is user controllable, an attacker could use this to inject an XSS vector into the $() selector sink. 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 ">". An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. 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. One example of an attribute which is thought to be safe is innerText. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. The logic which parses URLs in both execution and rendering contexts looks to be the same. XSS sinks are places where variables are placed into your webpage. Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. XSS Prevention & Mitigation. Get help and advice from our experts on all things Burp. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. Get started with Burp Suite Professional. If this isn't possible, then ensure the data is JavaScript encoded. Read the entire Acunetix Web Application Vulnerability Report. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. There are two distinct groups of cross-site scripting. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Tag helpers will also encode input you use in tag parameters. HTML Context refers to inserting a variable between two basic HTML tags like a