These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. 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. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). For example, you might need to close some existing elements before using your JavaScript payload. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. We will look at eval, href and dangerouslySetHTML vulnerabilities. Cross-Site Scripting (XSS) is a misnomer. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. For example. The rendered output would now become. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. DOM-based XSS Examples. Testing JavaScript execution sinks for DOM-based XSS is a little harder. The styling will not be rendered. The DOM is a programming interface. 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 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. Otherwise, again, your security efforts are void. 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. More info about Internet Explorer and Microsoft Edge. This cheat sheet provides guidance to prevent XSS vulnerabilities. For a comprehensive list, check out the DOMPurify allowlist. 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. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. XSS sinks are places where variables are placed into your webpage. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Parsing HTML input is difficult, if not impossible. A script within the later response contains a sink which then processes the data in an unsafe way. This can be done via a function such as: Read the entire Acunetix Web Application Vulnerability Report. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. There are two distinct groups of cross-site scripting. The only safe location for placing variables in JavaScript is inside a quoted data value. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. 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=. There are some further things to consider: Security professionals often talk in terms of sources and sinks. Free, lightweight web application security scanning for CI/CD. The example that follows illustrates using closures to avoid double JavaScript encoding. Save time/money. Trusted Types require you to process the data before passing it to the above sink functions. The good news is that if user input is handled properly at the foundation level (e.g. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. 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. //any code passed into lName is now executable. You might find that the source gets assigned to other variables. However, depending on the tag which innerText is applied, code can be executed. 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. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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%>. 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. The problem is that if companyName had the value "Johnson & Johnson". However the opposite is the case with HTML encoding. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. //The following does NOT work because of the encoded "(" and ")". . If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. 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. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. All other contexts are unsafe and you should not place variable data in them. Already got an account? WAFs also dont address the root cause of an XSS vulnerability. DOM Based Attacks. 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. As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. CSS Contexts refer to variables placed into inline CSS. If your code looked like the following, you would need to only double JavaScript encode input data. eval 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. 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). Based on this context, you need to refine your input to see how it is processed. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. element.SetAttribute () element [attribute]= Note that browsers behave differently with regards to URL-encoding, Chrome, Firefox, and Safari will URL-encode location.search and location.hash, while IE11 and Microsoft Edge (pre-Chromium) will not URL-encode these sources. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Other CSS Contexts are unsafe and you should not place variable data in them. This brings up an interesting design point. 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. Read more about DOM-based cross-site scripting. 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. Read about other types of cross-site scripting attacks. 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. 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. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Its critical to use quotation marks like " or ' to surround your variables. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. A rendering context is associated with the parsing of HTML tags and their attributes. 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. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. DOM XSS stands for Document Object Model-based Cross-site Scripting. Read more about DOM-based cross-site scripting. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. How common is DOM-based cross-site scripting? Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. There may be times you want to insert a value into JavaScript to process in your view. Safe list ranges are specified as Unicode code charts, not languages. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Cookie Attributes - These change how JavaScript and browsers can interact with cookies. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. 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. 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. Even newer versions of jQuery can still be vulnerable via the $() selector sink, provided you have full control over its input from a source that doesn't require a # prefix. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Some examples of DOM-based XSS attacks include: 1. Practise exploiting vulnerabilities on realistic targets. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Variables should only be placed in a CSS property value. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Trusted Types work by locking down the following risky sink functions. If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. 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. However, you may still find vulnerable code in the wild. How to detect DOM-based cross-site scripting? Now only JavaScript encoding on server side. Please insert your password to refresh your session. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. Before putting untrusted data inside an HTML element ensure it's HTML encoded. 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. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. //The following does NOT work because the event handler is being set to a string. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. On the client side, the HTTP response does not change but the script executes in malicious manner. For DOM XSS, the attack is injected into the application during runtime in the client directly. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. This is a Safe Sink and will automatically URL encode data in it. Make sure any attributes are fully quoted, same as JS and CSS. The best manual tools to start web security testing. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. If this is the case, you'll need to use the search function again to track these variables and see if they're passed to a sink. The line above could have possibly worked to render a link. 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. URL Contexts refer to variables placed into a URL. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. The DOM, or Document Object Model, is the structural format used to . Download the latest version of Burp Suite. If that isn't enough to keep in mind, you have to remember that encodings are lost when you retrieve them using the value attribute of a DOM element. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. Web Application Firewalls - These look for known attack strings and block them. 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, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. // is an example of untrusted data that was properly JavaScript encoded but still executes. The primary rule that you must follow to prevent DOM XSS is: sanitize all untrusted data, even if it is only used in client-side scripts. The most common one would be adding it to an href or src attribute of an tag. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. 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. No single technique will solve XSS. Cross-Site Scripting (XSS) is a misnomer. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. DOM based XSS vulnerabilities therefore have to be prevented on the client side. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. What would be displayed in the input text field would be "Johnson & Johnson". DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. It uses HTML attribute encoding rules whenever you use the @ directive. 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. Thankfully, many sinks where variables can be placed are safe. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC The #redir route is executed by another file, redir.html. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. How to prevent DOM-based cross-site scripting? This means you will need to use alternative elements like img or iframe. This means, that no data will be available in server logs. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. 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. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Output Encoding and HTML Sanitization help address those gaps. Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. I will show you three examples of DOM-based XSS attacks in this article. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. In the case above, JavaScript encoding does not mitigate against DOM based XSS. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. 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. 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.
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