Always pass untrusted input as a query string value. 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. DOM-based cross-site scripting is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. 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. 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. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. . Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Each encoder, Html, JavaScript and Url, must be configured separately. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. Using the right combination of defensive techniques is necessary to prevent XSS. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. It is the process of converting untrusted . Fewer XSS bugs appear in applications built with modern web frameworks. 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. Trusted Types work by locking down the following risky sink functions. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. 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. 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. DOM-based cross-site scripting attack DOM-based XSS is also sometimes called "type-0 XSS." It occurs when the XSS vector executes as a result of a DOM modification on a website in a user's browser. //The following DOES WORK because the encoded value is a valid variable name or function reference. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Login here. Avoid treating untrusted data as code or markup within JavaScript code. Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. It is a simple yet effective way to harvest passwords using only the victims browser. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. How to prevent DOM-based cross-site scripting? 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 ">". It is always a bad idea to use a user-controlled input in dangerous sources such as eval. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. 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 (#). This should never be used in combination with untrusted input as this will expose an XSS vulnerability. What's the difference between Pro and Enterprise Edition? //any code passed into lName is now executable. It's important to remember that some of these are also potential sources and sinks for DOM XSS. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. For DOM XSS, the attack is injected into the application during runtime in the client directly. In the case above, JavaScript encoding does not mitigate against DOM based XSS. 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. Based on this context, you need to refine your input to see how it is processed. 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. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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. . 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. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. Get your questions answered in the User Forum. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. 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. CSS Contexts refer to variables placed into inline CSS. Avoid populating the following methods with untrusted data. Misconceptions abound related to the proper encoding that is required. The problem is that if companyName had the value "Johnson & Johnson". The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Encoding libraries often have a EncodeForJavaScript or similar to support this function. In a DOM-based attacks, the HTTP response on the server side does not change. Trusted Types require you to process the data before passing it to the above sink functions. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. The DOM-based cross-site scripting requires the user to open an infected page. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. For details, see the Google Developers Site Policies. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. Don't use untrusted input as part of a URL path. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. 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. 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. How common is DOM-based cross-site scripting? A list of output encoding libraries is included in the appendix. The example that follows illustrates using closures to avoid double JavaScript encoding. Its critical to use quotation marks like " or ' to surround your variables. For example, websites often reflect URL parameters in the HTML response from the server. 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); }. DOM XSS stands for Document Object Model-based Cross-site Scripting. The data is subsequently read from the DOM by the web application and outputted to the browser. Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. //The following does NOT work because of the encoded ";". Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. 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. Don't mutate DOM directly. Otherwise, again, your security efforts are void. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. Working example (no HTML encoding): Normally encoded example (Does Not Work DNW): HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW): If HTML encoding followed the same semantics as JavaScript encoding. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. Summary. This could lead to an attack being added to a webpage.. for example. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. In some . Read about other types of cross-site scripting attacks. Thankfully, many sinks where variables can be placed are safe. Validation becomes more complicated when accepting HTML in user input. In many cases, JavaScript encoding does not stop attacks within an execution context. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. 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 #redir route is executed by another file, redir.html. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Tag helpers will also encode input you use in tag parameters. Web Application Firewalls - These look for known attack strings and block them. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. 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. Browsers change functionality and bypasses are being discovered regularly. View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. 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. This view outputs the contents of the untrustedInput variable. 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. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Event handlers such as onload and onerror can be used in conjunction with these elements. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Script manipulation: <script src> and setting text content of <script> elements. Variables should not be interpreted as code instead of text. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. Make sure any attributes are fully quoted, same as JS and CSS. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. Trusted Types force you to process a value. 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. 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. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Testing JavaScript execution sinks for DOM-based XSS is a little harder. In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: createElement () and assign property values with appropriate methods or properties such as node.textContent= or node.InnerText=. This is why you would need to HTML encode too. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. There are several methods and attributes which can be used to directly render HTML content within JavaScript. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. 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). All the Acunetix developers come with years of experience in the web security sphere. Want to track your progress and have a more personalized learning experience? This is because these sinks treat the variable as text and will never execute it. RULE #1 - HTML Escape then JavaScript Escape Before Inserting Untrusted Data into HTML Subcontext within the Execution Context, RULE #2 - JavaScript Escape Before Inserting Untrusted Data into HTML Attribute Subcontext within the Execution Context, RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context, RULE #4 - JavaScript Escape Before Inserting Untrusted Data into the CSS Attribute Subcontext within the Execution Context, RULE #5 - URL Escape then JavaScript Escape Before Inserting Untrusted Data into URL Attribute Subcontext within the Execution Context, RULE #6 - Populate the DOM using safe JavaScript functions or properties, RULE #7 - Fixing DOM Cross-site Scripting Vulnerabilities, Guidelines for Developing Secure Applications Utilizing JavaScript, GUIDELINE #1 - Untrusted data should only be treated as displayable text, GUIDELINE #2 - Always JavaScript encode and delimit untrusted data as quoted strings when entering the application when building templated JavaScript, GUIDELINE #3 - Use document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar to build dynamic interfaces, GUIDELINE #4 - Avoid sending untrusted data into HTML rendering methods, GUIDELINE #5 - Avoid the numerous methods which implicitly eval() data passed to it, Utilizing an Enclosure (as suggested by Gaz), GUIDELINE #6 - Use untrusted data on only the right side of an expression, GUIDELINE #7 - When URL encoding in DOM be aware of character set issues, GUIDELINE #8 - Limit access to object properties when using object[x] accessors, GUIDELINE #9 - Run your JavaScript in a ECMAScript 5 canopy or sandbox, GUIDELINE #10 - Don't eval() JSON to convert it to native JavaScript objects, Common Problems Associated with Mitigating DOM Based XSS, Insecure Direct Object Reference Prevention, Creative Commons Attribution 3.0 Unported License.