dom based cross site scripting prevention

Parsing HTML input is difficult, if not impossible. Encode all characters with the %HH encoding format. Learn more about types of cross-site scripting attacks It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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. 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. There may be times you want to insert a value into JavaScript to process in your view. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. The styling will not be rendered. Don't mutate DOM directly. 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. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. - owasp-CheatSheetSeries . A list of output encoding libraries is included in the appendix. It uses HTML attribute encoding rules whenever you use the @ directive. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. One example of an attribute which is thought to be safe is innerText. 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 uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). 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. Now only JavaScript encoding on server side. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. Read more about DOM-based cross-site scripting. 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). Reduce the DOM XSS attack surface of your application. Get started with Burp Suite Professional. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. Accelerate penetration testing - find more bugs, more quickly. Output Encoding. Content Security Policy - An allowlist that prevents content being loaded. DOM-based XSS is a kind of XSS occurring entirely on the client-side. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. 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. Misconceptions abound related to the proper encoding that is required. Do your applications use this vulnerable package? We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. The DOM-based cross-site scripting requires the user to open an infected page. Customization of the safe list only affects encoders sourced via DI. 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. 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. 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. Always pass untrusted input as a query string value. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. 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. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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. CSS is surprisingly powerful and has been used for many types of attacks. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. 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. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. Consider adopting the following controls in addition to the above. To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Get help and advice from our experts on all things Burp. 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. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . DOM Based Attacks. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. There will be times where you need to do something outside the protection provided by your framework. Avoid populating the following methods with untrusted data. There are 3 primary types of cross-site scripting: DOM-based XSS. For each potential source, such as location, you first need to find cases within the page's JavaScript code where the source is being referenced. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. Output Encoding and HTML Sanitization help address those gaps. XSS is one of the most common and dangerous web vulnerabilities, and it is . 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. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. Policies are factories for Trusted Types that enforce certain security rules on their input: This code creates a policy called myEscapePolicy that can produce TrustedHTML objects via its createHTML() function. eval Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Trusted Types require you to process the data before passing it to the above sink functions. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". . If you're using JavaScript to change a CSS property, look into using style.property = x. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. This document only discusses JavaScript bugs which lead to XSS. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Trusted Types work by locking down the following risky sink functions. 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. This fact makes it more difficult to maintain web application security. HTML tag elements are well defined and do not support alternate representations of the same tag. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Get the latest content on web security in your inbox each week. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Already got an account? The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. Some pure DOM-based vulnerabilities are self-contained within a single page. A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. 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). A list of safe HTML attributes is provided in the Safe Sinks section. What's the difference between Pro and Enterprise Edition? 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. For instance, jQuery's attr() function can change the attributes of DOM elements. Get the latest content on web security in your inbox each week. For example: To make dynamic updates to HTML in the DOM safe, we recommend: The HTML attribute subcontext within the execution context is divergent from the standard encoding rules. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. This is a Safe Sink and will automatically URL encode data in it. //The following does NOT work because the event handler is being set to a string. DOM-based cross-site scripting is the de-facto name for XSS bugs that are the result of active browser-side content on a page, typically JavaScript, obtaining user input and then doing something unsafe with it, leading to the execution of injected code. In DOM-based cross-site scripting, the HTML source code and response of the attack . Record your progression from Apprentice to Expert. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. 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. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. 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. Before putting untrusted data into a URL query string ensure it's URL encoded. At a basic level XSS works by tricking your application into inserting a