dom based cross site scripting prevention dom based cross site scripting prevention

The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. 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. Accelerate penetration testing - find more bugs, more quickly. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. All the Acunetix developers come with years of experience in the web security sphere. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. 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). Variables should only be placed in a CSS property value. 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. 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. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. eval Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Cross-Site Scripting (XSS) is a misnomer. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. For details, see the Google Developers Site Policies. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. If this isn't possible, then ensure the data is JavaScript encoded. your framework), you should be able to mitigate all XSS vulnerabilities. CSS is surprisingly powerful and has been used for many types of attacks. The styling will not be rendered. Use URL Encoding for these scenarios. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. RULE #1 - HTML Escape then JavaScript Escape Before Inserting Untrusted Data into HTML Subcontext within the Execution Context, RULE #2 - JavaScript Escape Before Inserting Untrusted Data into HTML Attribute Subcontext within the Execution Context, RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context, RULE #4 - JavaScript Escape Before Inserting Untrusted Data into the CSS Attribute Subcontext within the Execution Context, RULE #5 - URL Escape then JavaScript Escape Before Inserting Untrusted Data into URL Attribute Subcontext within the Execution Context, RULE #6 - Populate the DOM using safe JavaScript functions or properties, RULE #7 - Fixing DOM Cross-site Scripting Vulnerabilities, Guidelines for Developing Secure Applications Utilizing JavaScript, GUIDELINE #1 - Untrusted data should only be treated as displayable text, GUIDELINE #2 - Always JavaScript encode and delimit untrusted data as quoted strings when entering the application when building templated JavaScript, GUIDELINE #3 - Use document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar to build dynamic interfaces, GUIDELINE #4 - Avoid sending untrusted data into HTML rendering methods, GUIDELINE #5 - Avoid the numerous methods which implicitly eval() data passed to it, Utilizing an Enclosure (as suggested by Gaz), GUIDELINE #6 - Use untrusted data on only the right side of an expression, GUIDELINE #7 - When URL encoding in DOM be aware of character set issues, GUIDELINE #8 - Limit access to object properties when using object[x] accessors, GUIDELINE #9 - Run your JavaScript in a ECMAScript 5 canopy or sandbox, GUIDELINE #10 - Don't eval() JSON to convert it to native JavaScript objects, Common Problems Associated with Mitigating DOM Based XSS, Insecure Direct Object Reference Prevention, Creative Commons Attribution 3.0 Unported License. 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. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Please refer to the list below for details. DOM-based XSS Examples. DOM-based XSS is an advanced XSS attack. The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. In those cases, create a Trusted Type object yourself. There are 3 primary types of cross-site scripting: DOM-based XSS. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. 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. How to detect DOM-based cross-site scripting? 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. For each location where your string appears within the DOM, you need to identify the context. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. *Encoder.Default then the default, Basic Latin only safelist will be used. 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. 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. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. Reduce risk. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Event handlers such as onload and onerror can be used in conjunction with these elements. element.SetAttribute () element [attribute]= 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. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. The HTML encoded value above is still executable. 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. 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. Therefore, the primary recommendation is to avoid including untrusted data in this context. 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. You should apply HTML attribute encoding to variables being placed in most HTML attributes. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Do your applications use this vulnerable package? Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. 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. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. This is where Output Encoding and HTML Sanitization are critical. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). Read the entire Acunetix Web Application Vulnerability Report. 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 brings up an interesting design point. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. 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); }. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Fewer XSS bugs appear in applications built with modern web frameworks. The rendered output would now become. 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. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The third cross site scripting attack occurs entirely in the browser.