The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Trusted Types force you to process a value. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. Read more about DOM-based cross-site scripting. Get help and advice from our experts on all things Burp. 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. 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. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. This is because these sinks treat the variable as text and will never execute it. 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. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. For example: To make dynamic updates to HTML in the DOM safe, we recommend: The HTML attribute subcontext within the execution context is divergent from the standard encoding rules. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. Encode all characters with the %HH encoding format. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against XSS. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. When this happens, a script on the web page selects the URL variable and executes the code it contains. //any code passed into lName is now executable. 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. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. Web Application Firewalls - These look for known attack strings and block them. 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. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. It's important to remember that some of these are also potential sources and sinks for DOM XSS. Read the entire Acunetix Web Application Vulnerability Report. Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. 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. There are 3 primary types of cross-site scripting: DOM-based XSS. For DOM XSS, the attack is injected into the application during runtime in the client directly. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. 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. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. If you're using JavaScript to change a CSS property, look into using style.property = x. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. This is common when you want users to be able to customize the look and feel of their webpages. This means you will need to use alternative elements like img or iframe. Its critical to use quotation marks like " or ' to surround your variables. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. eval This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. XSS sinks are places where variables are placed into your webpage. 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. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. 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. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Acunetix developers and tech agents regularly contribute to the blog. Consider adopting the following controls in addition to the above. 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. See how our software enables the world to secure the web. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Use URL Encoding for these scenarios. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. In many cases, JavaScript encoding does not stop attacks within an 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. Normally executing JavaScript from a CSS context required either passing javascript:attackCode() to the CSS url() method or invoking the CSS expression() method passing JavaScript code to be directly executed. 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. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. HTML tag elements are well defined and do not support alternate representations of the same tag. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. . 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. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. On the client side, the HTTP response does not change but the script executes in malicious manner. 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. 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). 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. For details, see the Google Developers Site Policies. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. How to prevent DOM-based cross-site scripting? HTML Context refers to inserting a variable between two basic HTML tags like a
or . For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. 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. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Learn more about types of cross-site scripting attacks Any variable that does not go through this process is a potential weakness. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Free, lightweight web application security scanning for CI/CD. 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. 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. No single technique will solve XSS. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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 ">". DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Use a trusted and verified library to escape HTML inputs. 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. How common is DOM-based cross-site scripting? It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. These types of attacks typically occur as a result . OWASP recommends DOMPurify for HTML Sanitization. In the case above, JavaScript encoding does not mitigate against DOM based XSS. Read about other types of cross-site scripting attacks. A list of safe HTML attributes is provided in the Safe Sinks section. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. 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. If A is double JavaScript encoded then the following if check will return false. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. The rendered output would now become. 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 happens when data from a user controlled, Most of the violations like this can also be detected by running a code linter or, If the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. For example, websites often reflect URL parameters in the HTML response from the server. DOM XSS stands for Document Object Model-based Cross-site Scripting. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. This view outputs the contents of the untrustedInput variable. We are looking for web developers to participate in user research, product testing, discussion groups and more. 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. 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. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. It is the process of converting untrusted . If you pollute a river, it'll flow downstream somewhere. document.CreateTextNode () and append it in the appropriate DOM location. 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. If you directly access an encoder via System.Text.Encodings.Web. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Trusted Types heavily reduce the DOM XSS attack surface of your application. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. 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). With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. There are some further things to consider: Security professionals often talk in terms of sources and sinks. 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. A list of output encoding libraries is included in the appendix. Level up your hacking and earn more bug bounties. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. A Computer Science portal for geeks. WAFs also dont address the root cause of an XSS vulnerability. This document only discusses JavaScript bugs which lead to XSS. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. In those cases, create a Trusted Type object yourself. 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. However the opposite is the case with HTML encoding. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. 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. 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=. Want to track your progress and have a more personalized learning experience? 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. Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn't properly filter user input. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. 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. At a basic level XSS works by tricking your application into inserting a