XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. For example. Any application is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can develop from source to sink. For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). One example of an attribute which is thought to be safe is innerText. Testing JavaScript execution sinks for DOM-based XSS is a little harder. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. For example.. An attacker could modify data that is rendered as $varUnsafe. Thankfully, many sinks where variables can be placed are safe. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. Don't mutate DOM directly. DOM based XSS vulnerabilities therefore have to be prevented on the client side. The name originated from early versions of the attack where stealing data cross-site was the primary focus. 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. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. Each variable in a web application needs to be protected. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. 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. Input validation. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. Sometimes you can't change the offending code. element.SetAttribute () element [attribute]= Developers should use the following prevention steps to avoid introducing XSS into their application. All other contexts are unsafe and you should not place variable data in them. How common is DOM-based cross-site scripting? The best way to fix DOM based cross-site scripting is to use the right output method (sink). If A is double JavaScript encoded then the following if check will return false. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Limit access to object properties when using object[x] accessors (Mike Samuel). Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). If you must, the following examples describe some approaches that do and do not work. 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. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. 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). It also enables you to easily search your data without having to encode values before searching and allows you to take advantage of any changes or bug fixes made to encoders. There are a couple of options for fixing a Trusted Type violation. 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. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. //The following does NOT work because the event handler is being set to a string. 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. // is an example of untrusted data that was properly JavaScript encoded but still executes. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. Here are some examples of encoded values for specific characters. Use a trusted and verified library to escape HTML inputs. Catch critical bugs; ship more secure software, more quickly. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. The DOM-based cross-site scripting requires the user to open an infected page. Information on ordering, pricing, and more. A list of output encoding libraries is included in the appendix. No single technique will solve XSS. Sometimes users need to author HTML. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. 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. . *Encoder.Default then the default, Basic Latin only safelist will be used. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. However, depending on the tag which innerText is applied, code can be executed. 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 . However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. It is the process of converting untrusted . If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. . Output encoding is the primary defense against cross-site scripting vulnerabilities. Parsing HTML input is difficult, if not impossible. . You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Get started with Burp Suite Professional. If you have to use user input on your page, always use it in the text context, never as HTML tags or any other potential code. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. What would be displayed in the input text field would be "Johnson & Johnson". 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). Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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). 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. . 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. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. There are some further things to consider: Security professionals often talk in terms of sources and sinks. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. //The following does NOT work because of the encoded "(" and ")". 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. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. If your code looked like the following, you would need to only double JavaScript encode input data. This document only discusses JavaScript bugs which lead to XSS. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. This is common when you want users to be able to customize the look and feel of their webpages. Encode all characters using the \xHH format. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. 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. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. The most common one would be adding it to an href or src attribute of an tag. Encoding libraries often have a EncodeForJavaScript or similar to support this function. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. As with all other Cross-site Scripting (XSS) vulnerabilities, this type of attack also relies on insecure handling of user input on an HTML page. 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. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. 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. Before putting untrusted data into a URL query string ensure it's URL encoded. You need to work through each available source in turn, and test each one individually. Some examples of DOM-based XSS attacks include: 1. This cheat sheet provides guidance to prevent XSS vulnerabilities. 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. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. 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. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. The best manual tools to start web security testing. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. DOM-based XSS Examples. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. We will look at eval, href and dangerouslySetHTML vulnerabilities. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. 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). Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. Consider adopting the following controls in addition to the above. In a DOM-based attacks, the HTTP response on the server side does not change. 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. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. The styling will not be rendered. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. DOM-based XSS: In this type of attack, the attacker injects malicious code into a web page that is executed on the client-side within the Document Object Model (DOM) of the web page. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. The #redir route is executed by another file, redir.html. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Browsers change functionality and bypasses are being discovered regularly. For a comprehensive list, check out the DOMPurify allowlist. Accelerate penetration testing - find more bugs, more quickly. 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. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. In other words, add a level of indirection between untrusted input and specified object properties. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. We want to hear from you! DOM-based XSS vulnerabilities usually arise when JavaScript takes data from an attacker-controllable source, such as the URL, and passes it to a sink that supports dynamic code execution, such as eval() or innerHTML. 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. //any code passed into lName is now executable. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Any variable that does not go through this process is a potential weakness. Already got an account? As we use reCAPTCHA, you need to be able to access Google's servers to use this function. For DOM XSS, the attack is injected into the application during runtime in the client directly. There may be times you want to insert a value into JavaScript to process in your view. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Variables should not be interpreted as code instead of text. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. For details, see the Google Developers Site Policies. URL Contexts refer to variables placed into a URL. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. 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. Validation can be a useful tool in limiting XSS attacks. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Use only safe functions like document.innerText and document.textContent. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. \u0064\u006f\u0063\u0075\u006d\u0065\u006e\u0074, \u0077\u0072\u0069\u0074\u0065\u006c\u006e, "\u0048\u0065\u006c\u006c\u006f\u0020\u0057\u006f\u0072\u006c\u0064", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0031\u0029", "url(<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(companyName))%>)", '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(userRelativePath))%>', "<%= Encode.forJavaScript(untrustedData) %>", "<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>", "customFunction('<%=doubleJavaScriptEncodedData%>', y)", //HTML encoding is happening in JavaScript, "javascript:myFunction('<%=untrustedData%>', 'test');", "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(untrustedData)) %>', 'test');",