dom based cross site scripting preventiondom based cross site scripting prevention

DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). 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. It's important to remember that some of these are also potential sources and sinks for DOM XSS. Output encoding is not perfect. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. For a comprehensive list, check out the DOMPurify allowlist. However, you may still find vulnerable code in the wild. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. In some . 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). //any code passed into lName is now executable. 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. 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. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. 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. 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. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. The best way to fix DOM based cross-site scripting is to use the right output method (sink). This is because these sinks treat the variable as text and will never execute it. Automatic encoding and escaping functions are built into most frameworks. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. Validation can be a useful tool in limiting XSS attacks. Encoding libraries often have a EncodeForJavaScript or similar to support this function. Consider adopting the following controls in addition to the above. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. Use a CSP as an additional layer of defense and have a look at the. I will show you three examples of DOM-based XSS attacks in this article. Want to track your progress and have a more personalized learning experience? 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. Summary. 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=. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. - owasp-CheatSheetSeries . Safe list ranges are specified as Unicode code charts, not languages. It will not always prevent XSS. The HTML encoded value above is still executable. The Unicode standard has a list of code charts you can use to find the chart containing your characters. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. Please note, element.setAttribute is only safe for a limited number of attributes. Practise exploiting vulnerabilities on realistic targets. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. 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. The example that follows illustrates using closures to avoid double JavaScript encoding. We want to hear from you! Don't use untrusted input as part of a URL path. Accelerate penetration testing - find more bugs, more quickly. Always pass untrusted input as a query string value. 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. The best manual tools to start web security testing. The only safe location for placing variables in JavaScript is inside a quoted data value. There will be situations where you use a URL in different contexts. HTML tag elements are well defined and do not support alternate representations of the same tag. 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. It is also impossible to protect against such client-side attacks using WAFs. Use URL Encoding for these scenarios. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. 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 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. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). Fewer XSS bugs appear in applications built with modern web frameworks. Trusted Types give you the tools to write, security review, and maintain applications free of DOM XSS vulnerabilities by making the dangerous web API functions secure by default. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Tag helpers will also encode input you use in tag parameters. This is a Safe Sink and will automatically CSS encode data in it. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. Limit access to object properties when using object[x] accessors (Mike Samuel). The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. DOM-based XSS is a kind of XSS occurring entirely on the client-side. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. In a DOM-based attacks, the HTTP response on the server side does not change. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. At a basic level XSS works by tricking your application into inserting a