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

Each variable used in the user interface should be passed through an output encoding function. DOM based XSS vulnerabilities therefore have to be prevented on the client side. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. Trusted Types work by locking down the following risky sink functions. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. Get help and advice from our experts on all things Burp. This is because these sinks treat the variable as text and will never execute it. More info about Internet Explorer and Microsoft Edge. 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. 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. The problem is that if companyName had the value "Johnson & Johnson". Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. If A is double JavaScript encoded then the following if check will return false. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. This means you will need to use alternative elements like img or iframe. If you must, the following examples describe some approaches that do and do not work. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. Don't mutate DOM directly. Thankfully, many sinks where variables can be placed are safe. If this isn't possible, then ensure the data is JavaScript encoded. element.SetAttribute () element [attribute]= In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. CSS Contexts refer to variables placed into inline CSS. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. The DOM-based cross-site scripting requires the user to open an infected page. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). 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 information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. One of the simplest ways of doing this is to deliver your exploit via an iframe: In this example, the src attribute points to the vulnerable page with an empty hash value. Please insert your password to refresh your session. 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. 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 a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. placed in an HTML Attribute. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. The DOM is a programming interface. . To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. DOM-based XSS: DOM-based XSS occurs when an . You might find that the source gets assigned to other variables. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. 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. 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. 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%>. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. 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. 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. Encoding libraries often have a EncodeForJavaScript or similar to support this function. Get started with Burp Suite Enterprise Edition. 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. Once you've found where the source is being read, you can use the JavaScript debugger to add a break point and follow how the source's value is used. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Output Encoding. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. It is the process of converting untrusted . The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". 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). Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". For details, see the Google Developers Site Policies. Trusted Types require you to process the data before passing it to the above sink functions. Accelerate penetration testing - find more bugs, more quickly. CSS is surprisingly powerful and has been used for many types of attacks. Enhance security monitoring to comply with confidence. 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. Before putting untrusted data inside an HTML element ensure it's HTML encoded. 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). 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. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. It uses HTML attribute encoding rules whenever you use the @ directive. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. If you're using JavaScript to change a CSS property, look into using style.property = x. How common is DOM-based cross-site scripting? All other contexts are unsafe and you should not place variable data in them. 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. 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). In DOM-based cross-site scripting, the HTML source code and response of the attack . There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Learn the details here including XSS prevention methods. The styling will not be rendered. 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. 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. Cross-Site Scripting (XSS) is a misnomer. 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. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. . Validation becomes more complicated when accepting HTML in user input. *Encoder.Default then the default, Basic Latin only safelist will be used. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. A list of safe HTML attributes is provided in the Safe Sinks section. The logic which parses URLs in both execution and rendering contexts looks to be the same. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? The data is subsequently read from the DOM by the web application and outputted to the browser. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. This means, that no data will be available in server logs. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. On the client side, the HTTP response does not change but the script executes in malicious manner. The primary difference is where the attack is injected into the application. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. Free, lightweight web application security scanning for CI/CD. Semgrep rule to identify above dom xss link. It is an informational message with a simple alert. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Working example (no HTML encoding): Normally encoded example (Does Not Work DNW): HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW): If HTML encoding followed the same semantics as JavaScript encoding. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. 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. The good news is that if user input is handled properly at the foundation level (e.g. HTML encoding takes characters such as < and changes them into a safe form like < Before putting untrusted data into an HTML attribute ensure it's HTML encoded. 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. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. For instance, jQuery's attr() function can change the attributes of DOM elements. - owasp-CheatSheetSeries . In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. There are 3 primary types of cross-site scripting: DOM-based XSS. Want to track your progress and have a more personalized learning experience? However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Input validation. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. For example, using the default configuration you might use a Razor HtmlHelper like so; When you view the source of the web page you will see it has been rendered as follows, with the Chinese text encoded; To widen the characters treated as safe by the encoder you would insert the following line into the ConfigureServices() method in startup.cs; This example widens the safe list to include the Unicode Range CjkUnifiedIdeographs. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. We are looking for web developers to participate in user research, product testing, discussion groups and more. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. 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. There are some further things to consider: Security professionals often talk in terms of sources and sinks. Its critical to use quotation marks like " or ' to surround your variables. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Read more about DOM-based cross-site scripting. How to detect DOM-based cross-site scripting? If you need to render different content, use innerText instead of innerHTML. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. 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. 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). For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. HTML Context refers to inserting a variable between two basic HTML tags like a

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