This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Fewer XSS bugs appear in applications built with modern web frameworks. The best way to fix DOM based cross-site scripting is to use the right output method (sink). In other words, add a level of indirection between untrusted input and specified object properties. The document.write sink works with script elements, so you can use a simple payload, such as the one below: Note, however, that in some situations the content that is written to document.write includes some surrounding context that you need to take account of in your exploit. Tag helpers will also encode input you use in tag parameters. See how our software enables the world to secure the web. //The following does NOT work because the event handler is being set to a string. Read the entire Acunetix Web Application Vulnerability Report. Each encoder, Html, JavaScript and Url, must be configured separately. On the client side, the HTTP response does not change but the script executes in malicious manner. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. 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. . This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. your framework), you should be able to mitigate all XSS vulnerabilities. See what Acunetix Premium can do for you. Event handlers such as onload and onerror can be used in conjunction with these elements. 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). Content Security Policy - An allowlist that prevents content being loaded. When a browser is rendering HTML and any other associated content like CSS or JavaScript, it identifies various rendering contexts for the different kinds of input and follows different rules for each context. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. However, you may still find vulnerable code in the wild. Output Encoding. Other CSS Contexts are unsafe and you should not place variable data in them. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. The Unicode standard has a list of code charts you can use to find the chart containing your characters. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. It is also impossible to protect against such client-side attacks using WAFs. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. This could lead to an attack being added to a webpage.. for example. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. This is a Safe Sink and will automatically URL encode data in it. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. Read the entire Acunetix Web Application Vulnerability Report. 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. The #redir route is executed by another file, redir.html. 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. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. Cross-Site Scripting (XSS) is a misnomer. 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. Even newer versions of jQuery can still be vulnerable via the $() selector sink, provided you have full control over its input from a source that doesn't require a # prefix. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. Output Encoding and HTML Sanitization help address those gaps. In DOM-based cross-site scripting, the HTML source code and response of the attack . It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. DOM-based XSS simply means a cross-site scripting vulnerability that occurs in the DOM ( Document Object Model) of your site rather than in HTML. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. Variables should not be interpreted as code instead of text. The rendered output would now become. - owasp-CheatSheetSeries . Please note, element.setAttribute is only safe for a limited number of attributes. 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. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. There are numerous methods which implicitly eval() data passed to it that must be avoided. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. To test for DOM XSS in an HTML sink, place a random alphanumeric string into the source (such as location.search), then use developer tools to inspect the HTML and find where your string appears. 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. 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. The DOM-based cross-site scripting requires the user to open an infected page. There are a variety of sinks that are relevant to DOM-based vulnerabilities. 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. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. element.SetAttribute () element [attribute]= Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. DOM-based XSS is an advanced XSS attack. Here are some examples of encoded values for specific characters. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. For that, first create a policy. There are several methods and attributes which can be used to directly render HTML content within JavaScript. In this section, we'll describe DOM-based cross-site scripting (DOM XSS), explain how to find DOM XSS vulnerabilities, and talk about how to exploit DOM XSS with different sources and sinks. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. Validation can be a useful tool in limiting XSS attacks. 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. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . What would be displayed in the input text field would be "Johnson & Johnson". 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. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. To detect the possibility of a DOM XSS, you must simulate the attack from the client-side in the users browser using a web application scanner like Acunetix (with DOM-based XSS scanner functionality). Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? The third cross site scripting attack occurs entirely in the browser. Acunetix developers and tech agents regularly contribute to the blog. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Summary. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. Sometimes users need to author HTML. Testing JavaScript execution sinks for DOM-based XSS is a little harder. Already got an account? A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. 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. Encoding libraries often have a EncodeForJavaScript or similar to support this function. DOM-based XSS: DOM-based XSS occurs when an . Please note, it is always dangerous design to put untrusted data directly into a command execution context. Get started with Burp Suite Enterprise Edition. 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. 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 stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. Canonicalize input, URL Validation, Safe URL verification, Allow-list http and HTTPS URLs only (Avoid the JavaScript Protocol to Open a new Window), Attribute encoder. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. However, depending on the tag which innerText is applied, code can be executed. Trusted Types work by locking down the following risky sink functions. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. If these methods are provided with untrusted input, then an XSS vulnerability could result. Read about other types of cross-site scripting attacks. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Information on ordering, pricing, and more. 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. 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. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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). There are two ways to do this. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. 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%>. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. In those cases, create a Trusted Type object yourself. When other users load affected pages the attacker's scripts will run, enabling the attacker to steal cookies and session tokens, change the contents of the web page through DOM manipulation or redirect the browser to another page. Trusted Types force you to process a value. There are two distinct groups of cross-site scripting. 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. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. For a comprehensive list, check out the DOMPurify allowlist. The line above could have possibly worked to render a link. XSS sinks are places where variables are placed into your webpage. Each variable used in the user interface should be passed through an output encoding function. Free, lightweight web application security scanning for CI/CD. Accelerate penetration testing - find more bugs, more quickly. There are some further things to consider: Security professionals often talk in terms of sources and sinks. HTML Context refers to inserting a variable between two basic HTML tags like a