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 following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. This variable includes some characters which are used in XSS attacks, namely <, " and >. Download the latest version of Burp Suite. There are a couple of options for fixing a Trusted Type violation. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Always encode untrusted input before output, no matter what validation or sanitization has been performed. All the Acunetix developers come with years of experience in the web security sphere. Browsers change functionality and bypasses are being discovered regularly. 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. Trusted Types force you to process a value. 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 a simple yet effective way to harvest passwords using only the victims browser. 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. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. 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. OWASP TOP 10: Cross-site scripting (XSS) ~2023 | Udemy It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Avoid populating the following methods with untrusted data. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. The DOM-based cross-site scripting requires the user to open an infected page. 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. For instance, jQuery's attr() function can change the attributes of DOM elements. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. For example, you might need to close some existing elements before using your JavaScript payload. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. This is a Safe Sink and will automatically CSS encode data in it. We are looking for web developers to participate in user research, product testing, discussion groups and more. The reason why you only need to double JavaScript encode is that the customFunction function did not itself pass the input to another method which implicitly or explicitly called eval If firstName was passed to another JavaScript method which implicitly or explicitly called eval() then <%=doubleJavaScriptEncodedData%> above would need to be changed to <%=tripleJavaScriptEncodedData%>. 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. Please refer to the list below for details. . 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. I will show you three examples of DOM-based XSS attacks in this article. A Complete Guide To Cross Site Scripting - fas3c7.blogspot.com There may be times you want to insert a value into JavaScript to process in your view. Record your progression from Apprentice to Expert. 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. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. This view outputs the contents of the untrustedInput variable. How DOM Based XSS Attacks work - Bright Security The line above could have possibly worked to render a link. Consider adopting the following controls in addition to the above. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. Cross-site scripting XSS 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 (#). Each variable in a web application needs to be protected. //The following does NOT work because of the encoded "(" and ")". In some . OWASP recommends these in all circumstances. Avoid treating untrusted data as code or markup within JavaScript code. 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. 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. 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. 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. 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. The following is an example vulnerability which occurs in the JavaScript context and HTML subcontext: Let's look at the individual subcontexts of the execution context in turn. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Types of XSS (Cross-site Scripting) - Acunetix If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. 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. 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. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. For each location where your string appears within the DOM, you need to identify the context. What is XSS? Impact, Types, and Prevention - Bright Security We will look at eval, href and dangerouslySetHTML vulnerabilities. WAFs also dont address the root cause of an XSS vulnerability. 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. Now only JavaScript encoding on server side. Script manipulation: <script src> and setting text content of <script> elements. HTML tag elements are well defined and do not support alternate representations of the same tag. This means, that no data will be available in server logs. 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. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). Read the entire Acunetix Web Application Vulnerability Report. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. In other words, add a level of indirection between untrusted input and specified object properties. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. Automatic encoding and escaping functions are built into most frameworks. DOM-based XSS Examples. WSTG - v4.1 | OWASP Foundation Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. Developers should use the following prevention steps to avoid introducing XSS into their application. Cross Site Scripting (XSS) | OWASP Foundation Enhance security monitoring to comply with confidence. DOM-Based Cross-Site Scripting (DOM XSS) | Learn AppSec - Invicti This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. The data is subsequently read from the DOM by the web application and outputted to the browser. There are two distinct groups of cross-site scripting. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. 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. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. This cheat sheet provides guidance to prevent XSS vulnerabilities. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. In the case above, JavaScript encoding does not mitigate against DOM based XSS. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. Prevent Cross-Site Scripting (XSS) in ASP.NET Core 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. URL Contexts refer to variables placed into a URL. Don't use untrusted input as part of a URL path. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method.