If your code looked like the following, you would need to only double JavaScript encode input data. For example.. An attacker could modify data that is rendered as $varUnsafe. The name originated from early versions of the attack where stealing data cross-site was the primary focus. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Trusted Types require you to process the data before passing it to the above sink functions. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. For example, when your application passes a string to innerHTML, the browser sends the following report: This says that in https://my.url.example/script.js on line 39 innerHTML was called with the string beginning with <img src=x. Other CSS Contexts are unsafe and you should not place variable data in them. 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. No single technique will solve XSS. Stored XSS is considered the most damaging type of XSS attack. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. It is a simple yet effective way to harvest passwords using only the victims browser. 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. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. Read about other types of cross-site scripting attacks. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. For example, you might need to close some existing elements before using your JavaScript payload. 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. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. 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 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. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). 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. For example, Acunetix. Save time/money. Using the right combination of defensive techniques is necessary to prevent XSS. When this happens, a script on the web page selects the URL variable and executes the code it contains. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. Before putting untrusted data inside an HTML element ensure it's HTML encoded. This is because these sinks treat the variable as text and will never execute it. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Cross-Site Scripting (XSS) is a misnomer. See what Acunetix Premium can do for you. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. In the case above, JavaScript encoding does not mitigate against DOM based XSS. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. 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. placed in an HTML Attribute. This is a Safe Sink and will automatically URL encode data in it. In DOM-based cross-site scripting, the HTML source code and response of the attack . //The following does NOT work because of the encoded "(" and ")". Information on ordering, pricing, and more. 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. Want to track your progress and have a more personalized learning experience? It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Read the entire Acunetix Web Application Vulnerability Report. 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. Customization of the safe list only affects encoders sourced via DI. This means, that no data will be available in server logs. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. 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. 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=. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. CSS is surprisingly powerful and has been used for many types of attacks. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) 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(). There are some further things to consider: Security professionals often talk in terms of sources and sinks. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. WAFs also dont address the root cause of an XSS vulnerability. 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. 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. There will be situations where you use a URL in different contexts. 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. The #redir route is executed by another file, redir.html. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. . If you pollute a river, it'll flow downstream somewhere. Record your progression from Apprentice to Expert. In those cases, create a Trusted Type object yourself. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Always encode untrusted input before output, no matter what validation or sanitization has been performed. 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. Each encoder, Html, JavaScript and Url, must be configured separately. Encode all characters using the \xHH format. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. Don't use untrusted input as part of a URL path. Please note, element.setAttribute is only safe for a limited number of attributes. There are two distinct groups of cross-site scripting. The only safe location for placing variables in JavaScript is inside a quoted data value. Its critical to use quotation marks like " or ' to surround your variables. XSS sinks are places where variables are placed into your webpage. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. \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');",