One example of an attribute which is thought to be safe is innerText. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). Before putting untrusted data inside an HTML element ensure it's HTML encoded. . In those cases, create a Trusted Type object yourself. In some . Get started with Burp Suite Professional. 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 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. (It's free!). Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. The #redir route is executed by another file, redir.html. On the client side, the HTTP response does not change but the script executes in malicious manner. However, depending on the tag which innerText is applied, code can be executed. If you directly access an encoder via System.Text.Encodings.Web. This document only discusses JavaScript bugs which lead to XSS. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. How to prevent DOM-based cross-site scripting? Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. We want to hear from you! Level up your hacking and earn more bug bounties. Variables should not be interpreted as code instead of text. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. This is where Output Encoding and HTML Sanitization are critical. When a browser is rendering HTML and any other associated content like CSS or JavaScript, it identifies various rendering contexts for the different kinds of input and follows different rules for each context. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. Get help and advice from our experts on all things Burp. 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. Variables should only be placed in a CSS property value. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. Common injection vectors include document.url, document.location, and document.referrer objects. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. 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. Want to track your progress and have a more personalized learning experience? HTML Context refers to inserting a variable between two basic HTML tags like a
or . HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. 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. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Cross-Site Scripting (XSS) is a misnomer. 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. Sometimes users need to author HTML. Read more about DOM-based cross-site scripting. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. For details, see the Google Developers Site Policies. 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. In the case above, JavaScript encoding does not mitigate against DOM based XSS. It uses HTML attribute encoding rules whenever you use the @ directive. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. DOM-based XSS: DOM-based XSS occurs when an . Limit access to object properties when using object[x] accessors (Mike Samuel). 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. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. 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. When this happens, a script on the web page selects the URL variable and executes the code it contains. This view outputs the contents of the untrustedInput variable. Don't use untrusted input as part of a URL path. There are numerous methods which implicitly eval() data passed to it that must be avoided. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. 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. Before putting untrusted data into a URL query string ensure it's URL encoded. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. For example.. An attacker could modify data that is rendered as $varUnsafe. Browsers change functionality and bypasses are being discovered regularly. If you sanitize content and then modify it afterwards, you can easily void your security efforts. 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. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. 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. DOM-based XSS is a kind of XSS occurring entirely on the client-side. Read the entire Acunetix Web Application Vulnerability Report. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. Validation can be a useful tool in limiting XSS attacks. Developers should use the following prevention steps to avoid introducing XSS into their application. 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. You might find that the source gets assigned to other variables. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. 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. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". Use only safe functions like document.innerText and document.textContent. Please insert your password to refresh your session. Trusted Types heavily reduce the DOM XSS attack surface of your application. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. For example, Acunetix. Trusted Types work by locking down the following risky sink functions. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. A rendering context is associated with the parsing of HTML tags and their attributes. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. document.CreateTextNode () and append it in the appropriate DOM location. Also, XSS attacks always execute in the browser. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. No single technique will solve XSS. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. 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. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. Use a CSP as an additional layer of defense and have a look at the. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. 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. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Each parser has distinct and separate semantics in the way they can possibly execute script code which make creating consistent rules for mitigating vulnerabilities in various contexts difficult. For example, if your string appears within a double-quoted attribute then try to inject double quotes in your string to see if you can break out of the attribute. There are 3 primary types of cross-site scripting: DOM-based XSS. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. How common is DOM-based cross-site scripting? eval If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . Scale dynamic scanning. Event handlers such as onload and onerror can be used in conjunction with these elements. DOM Based Attacks. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. If you must, the following examples describe some approaches that do and do not work. The data is subsequently read from the DOM by the web application and outputted to the browser. 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. It is also impossible to protect against such client-side attacks using WAFs. 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 (#). 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. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. 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. Encode all characters with the %HH encoding format. The DOM is a programming interface. Read more about DOM-based cross-site scripting. A list of output encoding libraries is included in the appendix. 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. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. Each variable in a web application needs to be protected. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. Enhance security monitoring to comply with confidence. Learn more about types of cross-site scripting attacks These methods constitute the HTML Subcontext within the Execution Context. Fewer XSS bugs appear in applications built with modern web frameworks. 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.