In many cases, JavaScript encoding does not stop attacks within an execution context. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. DOM based XSS vulnerabilities therefore have to be prevented on the client side. There are 3 primary types of cross-site scripting: DOM-based XSS. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. The styling will not be rendered. Content Security Policy - An allowlist that prevents content being loaded. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. //The following does NOT work because of the encoded ";". This means you will need to use alternative elements like img or iframe. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. It is the process of converting untrusted . Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. Output encoding is the primary defense against cross-site scripting vulnerabilities. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) There are some further things to consider: Security professionals often talk in terms of sources and sinks. Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. After the page's JavaScript applies this malicious URL to the back link's href, clicking on the back link will execute it: Another potential sink to look out for is jQuery's $() selector function, which can be used to inject malicious objects into the DOM. 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. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Validation can be a useful tool in limiting XSS attacks. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. 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. However the opposite is the case with HTML encoding. When this happens, a script on the web page selects the URL variable and executes the code it contains. Scale dynamic scanning. 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. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. 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. 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. 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. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. 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. 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. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. 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. For instance, jQuery's attr() function can change the attributes of DOM elements. Please refer to the list below for details. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. Don't use untrusted input as part of a URL path. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Output encoding here will prevent XSS, but it will break the intended functionality of the application. 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. Output Encoding and HTML Sanitization help address those gaps. This is where Output Encoding and HTML Sanitization are critical. How to detect DOM-based cross-site scripting? These methods constitute the HTML Subcontext within the Execution Context. There are two ways to do this. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. What's the difference between Pro and Enterprise Edition? Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. 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. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. The other alternative is using N-levels of encoding. In some . Otherwise, again, your security efforts are void. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. This is because these sinks treat the variable as text and will never execute it. An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. eval 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. 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. Practise exploiting vulnerabilities on realistic targets. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. In a DOM-based attacks, the HTTP response on the server side does not change. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. Use URL Encoding for these scenarios. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. 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. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. 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. // is an example of untrusted data that was properly JavaScript encoded but still executes. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. The complication is compounded by the differing meanings and treatment of encoded values within each subcontext (HTML, HTML attribute, URL, and CSS) within the execution context. 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 the simplest ways of doing this is to deliver your exploit via an iframe: In this example, the src attribute points to the vulnerable page with an empty hash value. Please note, element.setAttribute is only safe for a limited number of attributes. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. This is a Safe Sink and will automatically CSS encode data in it. It's important to remember that some of these are also potential sources and sinks for DOM XSS. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). Other CSS Contexts are unsafe and you should not place variable data in them. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. 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. Reduce the DOM XSS attack surface of your application. Read the entire Acunetix Web Application Vulnerability Report. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. 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. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. In these cases, HTML Sanitization should be used. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. 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. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. The HTML parser of the rendering context dictates how data is presented and laid out on the page and can be further broken down into the standard contexts of HTML, HTML attribute, URL, and CSS. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. In JavaScript code, the main context is JavaScript but with the right tags and context closing characters, an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods. DOM-based XSS is a kind of XSS occurring entirely on the client-side. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. 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. Doing so encourages designs in which the security rules are close to the data that they process, where you have the most context to correctly sanitize the value. 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. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. A script within the later response contains a sink which then processes the data in an unsafe way. Sometimes you can't change the offending code. 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. Thankfully, many sinks where variables can be placed are safe. The logic which parses URLs in both execution and rendering contexts looks to be the same. Get started with Burp Suite Professional. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). 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. The best manual tools to start web security testing. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Websites may also store data on the server and reflect it elsewhere. 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. Its the same with computer security. How common is DOM-based cross-site scripting? If that isn't enough to keep in mind, you have to remember that encodings are lost when you retrieve them using the value attribute of a DOM element. Want to track your progress and have a more personalized learning experience? This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Trusted Types force you to process a value. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. 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. Consider adopting the following controls in addition to the above. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. It is also impossible to protect against such client-side attacks using WAFs. placed in an HTML Attribute. You need to work through each available source in turn, and test each one individually. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. If your code looked like the following, you would need to only double JavaScript encode input data. 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. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. 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. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. 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. 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. 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. 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). Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. If you pollute a river, it'll flow downstream somewhere. Get the latest content on web security in your inbox each week. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. 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 is a Safe Sink and will automatically URL encode data in it. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. 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. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. This means, that no data will be available in server logs. So XSS has already been around for a while. 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(). The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Its critical to use quotation marks like " or ' to surround your variables. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. Make sure any attributes are fully quoted, same as JS and CSS. Output Encoding. OWASP recommends DOMPurify for HTML Sanitization. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Trusted Types work by locking down the following risky sink functions. 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. The most common one would be adding it to an href or src attribute of an tag. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. If this isn't possible, then ensure the data is JavaScript encoded. 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. These locations are known as dangerous contexts. The only safe location for placing variables in JavaScript is inside a quoted data value. At a basic level XSS works by tricking your application into inserting a