Acunetix developers and tech agents regularly contribute to the blog. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. The world's #1 web penetration testing toolkit. Any variable that does not go through this process is a potential weakness. 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. It is also impossible to protect against such client-side attacks using WAFs. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Otherwise, again, your security efforts are void. HTML Context refers to inserting a variable between two basic HTML tags like a
or . Other CSS Contexts are unsafe and you should not place variable data in them. // is an example of untrusted data that was properly JavaScript encoded but still executes. If you must, the following examples describe some approaches that do and do not work. There may be times you want to insert a value into JavaScript to process in your view. Limit access to object properties when using object[x] accessors (Mike Samuel). I will show you three examples of DOM-based XSS attacks in this article. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. Browsers change functionality and bypasses are being discovered regularly. Trusted Types force you to process a value. The Unicode standard has a list of code charts you can use to find the chart containing your characters. 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(). For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. The only safe location for placing variables in JavaScript is inside a quoted data value. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. 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). For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) 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. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. There are several methods and attributes which can be used to directly render HTML content within JavaScript. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. In other words, add a level of indirection between untrusted input and specified object properties. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. 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. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. 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 . This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Always encode untrusted input before output, no matter what validation or sanitization has been performed. 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. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. More info about Internet Explorer and Microsoft Edge. OWASP recommends these in all circumstances. The DOM is a programming interface. 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 ">". Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. DOM-based XSS simply means a cross-site scripting vulnerability that occurs in the DOM ( Document Object Model) of your site rather than in HTML. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. This variable includes some characters which are used in XSS attacks, namely <, " and >. Thankfully, many sinks where variables can be placed are safe. It is the process of converting untrusted . For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. Read the entire Acunetix Web Application Vulnerability Report. CSS is surprisingly powerful and has been used for many types of attacks. Therefore, the primary recommendation is to avoid including untrusted data in this context. If these methods are provided with untrusted input, then an XSS vulnerability could result. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Customization of the safe list only affects encoders sourced via DI. Level up your hacking and earn more bug bounties. Read more about DOM-based cross-site scripting. However, depending on the tag which innerText is applied, code can be executed. Finally, to fix the problem in our initial code, instead of trying to encode the output correctly which is a hassle and can easily go wrong we would simply use element.textContent to write it in a content like this: It does the same thing but this time it is not vulnerable to DOM based cross-site scripting vulnerabilities. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. 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. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. Get the latest content on web security in your inbox each week. DOM Based Attacks. 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. 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. URL Contexts refer to variables placed into a URL. . Reduce risk. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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=. Tag helpers will also encode input you use in tag parameters. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. These locations are known as dangerous contexts. Read the entire Acunetix Web Application Vulnerability Report. 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. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. 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. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. 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. In many cases, JavaScript encoding does not stop attacks within an execution context. Output Encoding. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Catch critical bugs; ship more secure software, more quickly. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. One example of an attribute which is thought to be safe is innerText. Content Security Policy - An allowlist that prevents content being loaded. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. Never rely on validation alone. Sometimes you can't change the offending code. In DOM-based cross-site scripting, the HTML source code and response of the attack . That said, developers need to be aware of problems that can occur when using frameworks insecurely such as: Understand how your framework prevents XSS and where it has gaps. 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. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. 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. Summary. 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. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. document.CreateTextNode () and append it in the appropriate DOM location. Get your questions answered in the User Forum. There are 3 primary types of cross-site scripting: DOM-based XSS. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. 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. This is common when you want users to be able to customize the look and feel of their webpages. - owasp-CheatSheetSeries . DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. XSS Prevention & Mitigation. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. All the Acunetix developers come with years of experience in the web security sphere. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response.
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