However the opposite is the case with HTML encoding. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Perhaps the non-conforming functionality is not needed anymore or can be rewritten in a modern way without using the error-prone functions?Don'tel.innerHTML = '<img src=xyz.jpg>'; Doel.textContent = '';const img = document.createElement('img');img.src = 'xyz.jpg';el.appendChild(img); Some libraries already generate Trusted Types that you can pass to the sink functions. 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. 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. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. Information on ordering, pricing, and more. . XSS is one of the most common and dangerous web vulnerabilities, and it is . Cross-Site Scripting (XSS) is a misnomer. In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template. If your code looked like the following, you would need to only double JavaScript encode input data. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. //any code passed into lName is now executable. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. 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 . Output encoding here will prevent XSS, but it will break the intended functionality of the application. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. 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. An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. The HTML encoded value above is still executable. Avoid populating the following methods with untrusted data. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. The best manual tools to start web security testing. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Accelerate penetration testing - find more bugs, more quickly. Learn the details here including XSS prevention methods. Consider adopting the following controls in addition to the above. Get the latest content on web security in your inbox each week. See what Acunetix Premium can do for you. 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. URL Contexts refer to variables placed into a URL. DOM-based XSS is an advanced XSS attack. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). For each location where your string appears within the DOM, you need to identify the context. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. 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. Sometimes users need to author HTML. (It's free!). DOM Based Attacks. Semgrep rule to identify above dom xss link. Variables should not be interpreted as code instead of text. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. For example: Modern web applications are typically built using a number of third-party libraries and frameworks, which often provide additional functions and capabilities for developers. This cheat sheet provides guidance to prevent XSS vulnerabilities. In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. 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. A Computer Science portal for geeks. Want to track your progress and have a more personalized learning experience? Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. 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. OWASP recommends these in all circumstances. Read the entire Acunetix Web Application Vulnerability Report. Please note, element.setAttribute is only safe for a limited number of attributes. Misconceptions abound related to the proper encoding that is required. 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. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). A list of safe HTML attributes is provided in the Safe Sinks section. Based on this context, you need to refine your input to see how it is processed. 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. In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. There are a couple of options for fixing a Trusted Type violation. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Output Encoding. 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%>. The name originated from early versions of the attack where stealing data cross-site was the primary focus. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. 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. How to detect DOM-based cross-site scripting? A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. There are two ways to do this. 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. 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. 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. Its the same with computer security. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. No single technique will solve XSS. Record your progression from Apprentice to Expert. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Your application can be vulnerable to both reflected/stored XSS and DOM XSS. These types of attacks typically occur as a result . Encode all characters with the %HH encoding format. Output encoding is the primary defense against cross-site scripting vulnerabilities. Download the latest version of Burp Suite. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. Acunetix developers and tech agents regularly contribute to the blog. Get started with Burp Suite Enterprise Edition. For a comprehensive list, check out the DOMPurify allowlist. 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. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. Scale dynamic scanning. Content Security Policy - An allowlist that prevents content being loaded. 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. This is why you would need to HTML encode too. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. 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. If you pollute a river, it'll flow downstream somewhere. //The following DOES WORK because the encoded value is a valid variable name or function reference. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. In DOM-based cross-site scripting, the HTML source code and response of the attack . "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". There will be situations where you use a URL in different contexts. It will not always prevent XSS. The best way to fix DOM based cross-site scripting is to use the right output method (sink). This variable includes some characters which are used in XSS attacks, namely <, " and >. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. If you must, the following examples describe some approaches that do and do not work. 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. Use a trusted and verified library to escape HTML inputs. In those cases, create a Trusted Type object yourself. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. For example. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. 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 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. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. More info about Internet Explorer and Microsoft Edge. Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. It uses HTML attribute encoding rules whenever you use the @ directive. So XSS has already been around for a while. Always pass untrusted input as a query string value. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. element.SetAttribute () element [attribute]= You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. We will look at eval, href and dangerouslySetHTML 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. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. Fewer XSS bugs appear in applications built with modern web frameworks. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. Event handlers such as onload and onerror can be used in conjunction with these elements. Developers should use the following prevention steps to avoid introducing XSS into their application. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Validation can be a useful tool in limiting XSS attacks. 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=. All of this code originates on the server, which means it is the application owner's responsibility to make it safe from XSS, regardless of the type of XSS flaw it is. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. 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. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. 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. 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. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. If you're using JavaScript to change a CSS property, look into using style.property = x. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Common injection vectors include document.url, document.location, and document.referrer objects. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Other CSS Contexts are unsafe and you should not place variable data in them. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. 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. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). When your application no longer produces violations, you can start enforcing Trusted Types: Voila! It is an informational message with a simple alert. 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. The third cross site scripting attack occurs entirely in the browser. 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). Please insert your password to refresh your session. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. 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. 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. We want to hear from you! An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. Encoding libraries often have a EncodeForJavaScript or similar to support this function. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. 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. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. 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. HTML Context refers to inserting a variable between two basic HTML tags like a

or . \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');",