dom based cross site scripting prevention

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. In the case above, JavaScript encoding does not mitigate against DOM based XSS. If you directly access an encoder via System.Text.Encodings.Web. This means, that no data will be available in server logs. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. The data is subsequently read from the DOM by the web application and outputted to the browser. 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 certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. OWASP recommends these in all circumstances. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. Use only safe functions like document.innerText and document.textContent. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Reduce the DOM XSS attack surface of your application. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. You need to work through each available source in turn, and test each one individually. See what Acunetix Premium can do for you. DOM-based XSS is a kind of XSS occurring entirely on the client-side. //The following DOES WORK because the encoded value is a valid variable name or function reference. Already got an account? 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. 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. Free, lightweight web application security scanning for CI/CD. The styling will not be rendered. XSS Prevention & Mitigation. //The following does NOT work because of the encoded "(" and ")". DOM XSS stands for Document Object Model-based Cross-site Scripting. One example of an attribute which is thought to be safe is innerText. 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 logic which parses URLs in both execution and rendering contexts looks to be the same. 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. 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=. 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. Ideally, the correct way to apply encoding and avoid the problem stated above is to server-side encode for the output context where data is introduced into the application. Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. The DOM-based cross-site scripting requires the user to open an infected page. In other words, add a level of indirection between untrusted input and specified object properties. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. Testing JavaScript execution sinks for DOM-based XSS is a little harder. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. When this happens, a script on the web page selects the URL variable and executes the code it contains. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Avoid treating untrusted data as code or markup within JavaScript code. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. 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 . In these cases, HTML Sanitization should be used. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Misconceptions abound related to the proper encoding that is required. These methods constitute the HTML Subcontext within the Execution Context. Use a trusted and verified library to escape HTML inputs. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. For example, websites often reflect URL parameters in the HTML response from the server. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. Developers should use the following prevention steps to avoid introducing XSS into their application. 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. Consider adopting the following controls in addition to the above. 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. The enterprise-enabled dynamic web vulnerability scanner. We are looking for web developers to participate in user research, product testing, discussion groups and more. For example, Acunetix. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. The name originated from early versions of the attack where stealing data cross-site was the primary focus. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. 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). Read about other types of cross-site scripting attacks. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). 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. Parsing HTML input is difficult, if not impossible. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. CSS Contexts refer to variables placed into inline CSS. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. document.CreateTextNode () and append it in the appropriate DOM location. Enhance security monitoring to comply with confidence. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. 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. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. It is also impossible to protect against such client-side attacks using WAFs. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. 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 will not always prevent XSS. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. Always encode untrusted input before output, no matter what validation or sanitization has been performed. 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. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. 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). In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Avoid populating the following methods with untrusted data. your framework), you should be able to mitigate all XSS vulnerabilities. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. Encoding libraries often have a EncodeForJavaScript or similar to support this function. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. The good news is that if user input is handled properly at the foundation level (e.g. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. The best manual tools to start web security testing. Trusted Types force you to process a value. 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. Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. This view outputs the contents of the untrustedInput variable. 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. 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. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. Other CSS Contexts are unsafe and you should not place variable data in them. 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. Common injection vectors include document.url, document.location, and document.referrer objects. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. 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. You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. Don't mutate DOM directly. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. *Encoder.Default then the default, Basic Latin only safelist will be used. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. This document only discusses JavaScript bugs which lead to XSS. Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. 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? There are 3 primary types of cross-site scripting: DOM-based XSS. If you must, the following examples describe some approaches that do and do not work. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. See Browser compatibility for up-to-date cross-browser support information.Key TermDOM-based cross-site scripting happens when data from a user controlled source (like user name, or redirect URL taken from the URL fragment) reaches a sink, which is a function like eval() or a property setter like .innerHTML, that can execute arbitrary JavaScript code. DOM-based XSS: DOM-based XSS occurs when an . In many cases, JavaScript encoding does not stop attacks within an execution context. Output encoding here will prevent XSS, but it will break the intended functionality of the application. The line above could have possibly worked to render a link. Reduce risk. 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. 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. 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 is a Safe Sink and will automatically CSS encode data in it. 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. Record your progression from Apprentice to Expert. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. Always pass untrusted input as a query string value. 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. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. If you pollute a river, it'll flow downstream somewhere. 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. //The following does NOT work because the event handler is being set to a string. Learn more about types of cross-site scripting attacks With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Validation can be a useful tool in limiting XSS attacks. 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). 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. It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. 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. This cheat sheet provides guidance to prevent XSS vulnerabilities. Each encoder, Html, JavaScript and Url, must be configured separately. Get help and advice from our experts on all things Burp. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. Get started with Burp Suite Professional. Never rely on validation alone. Summary. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. Each variable used in the user interface should be passed through an output encoding function. In many cases the context isn't always straightforward to discern. How common is DOM-based cross-site scripting? \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');",