dom based cross site scripting prevention

Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. . 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. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. Do your applications use this vulnerable package? // is an example of untrusted data that was properly JavaScript encoded but still executes. There are some further things to consider: Security professionals often talk in terms of sources and sinks. In other words, add a level of indirection between untrusted input and specified object properties. Get started with Burp Suite Professional. If you pollute a river, it'll flow downstream somewhere. Encode all characters using the \xHH format. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). There are 3 primary types of cross-site scripting: DOM-based XSS. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Variables should not be interpreted as code instead of text. How to detect DOM-based cross-site scripting? In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. 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. 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. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. 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. If you sanitize content and then modify it afterwards, you can easily void your security efforts. Information on ordering, pricing, and more. In those cases, create a Trusted Type object yourself. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. In DOM-based cross-site scripting, the HTML source code and response of the attack . 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. 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. 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. 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). The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Validation can be a useful tool in limiting XSS attacks. The document.write sink works with script elements, so you can use a simple payload, such as the one below: Note, however, that in some situations the content that is written to document.write includes some surrounding context that you need to take account of in your exploit. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. 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. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Reduce risk. Level up your hacking and earn more bug bounties. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. 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. 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. 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. 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. 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. You might find that the source gets assigned to other variables. 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. Login here. 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. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. Scale dynamic scanning. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. You should apply HTML attribute encoding to variables being placed in most HTML attributes. Cross-Site Scripting (XSS) is a misnomer. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. CSS Contexts refer to variables placed into inline CSS. 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. 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. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. In this section, we'll describe DOM-based cross-site scripting (DOM XSS), explain how to find DOM XSS vulnerabilities, and talk about how to exploit DOM XSS with different sources and sinks. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. Here are some examples of encoded values for specific characters. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. See what Acunetix Premium can do for you. 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. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Please refer to the list below for details. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. CSS is surprisingly powerful and has been used for many types of attacks. Record your progression from Apprentice to Expert. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. This fact makes it more difficult to maintain web application security. DOM-based cross-site scripting is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. 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. Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. There will be situations where you use a URL in different contexts. Catch critical bugs; ship more secure software, more quickly. 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. . If you need to render different content, use innerText instead of innerHTML. A Computer Science portal for geeks. Get the latest content on web security in your inbox each week. 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. 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). Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). What would be displayed in the input text field would be "Johnson & Johnson". Note how the payload is stored in the GET request, making it suitable for social engineering attacks. Don't use untrusted input as part of a URL path. Some pure DOM-based vulnerabilities are self-contained within a single page. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. When this happens, a script on the web page selects the URL variable and executes the code it contains. Avoid populating the following methods with untrusted data. Stored XSS is considered the most damaging type of XSS attack. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. 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 ">". Trusted Types force you to process a value. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. For example. Trusted Types heavily reduce the DOM XSS attack surface of your application. What's the difference between Pro and Enterprise Edition? HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. - owasp-CheatSheetSeries . JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. //The following does NOT work because of the encoded "(" and ")". The enterprise-enabled dynamic web vulnerability scanner. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. These methods constitute the HTML Subcontext within the Execution Context. DOM Based Attacks. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Sometimes you can't change the offending code. 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. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. This means, that no data will be available in server logs. DOMPurify supports Trusted Types and will return sanitized HTML wrapped in a TrustedHTML object such that the browser does not generate a violation.CautionIf the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. Acunetix developers and tech agents regularly contribute to the blog. 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. For example, you might need to close some existing elements before using your JavaScript payload. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. DOM-based cross-site scripting happens when data from a user controlled, Most of the violations like this can also be detected by running a code linter or, If the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! Read the entire Acunetix Web Application Vulnerability Report. Automatic encoding and escaping functions are built into most frameworks. All other contexts are unsafe and you should not place variable data in them. 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. For example.. An attacker could modify data that is rendered as $varUnsafe. 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. The Unicode standard has a list of code charts you can use to find the chart containing your characters. For DOM XSS, the attack is injected into the application during runtime in the client directly. 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. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . 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. So XSS has already been around for a while. Its the same with computer security. DOM based XSS vulnerabilities therefore have to be prevented on the client side. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. 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. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. Browsers change functionality and bypasses are being discovered regularly. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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. Save time/money. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. 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 data is subsequently read from the DOM by the web application and outputted to the browser. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. The most common one would be adding it to an href or src attribute of an tag. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. To test for DOM XSS in an HTML sink, place a random alphanumeric string into the source (such as location.search), then use developer tools to inspect the HTML and find where your string appears. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). This means you will need to use alternative elements like img or iframe. The following are some of the main sinks that can lead to DOM-XSS vulnerabilities: The following jQuery functions are also sinks that can lead to DOM-XSS vulnerabilities: In addition to the general measures described on the DOM-based vulnerabilities page, you should avoid allowing data from any untrusted source to be dynamically written to the HTML document. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. Use a trusted and verified library to escape HTML inputs. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Cross-Site Scripting (XSS) is a misnomer. For a comprehensive list, check out the DOMPurify allowlist. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. This is common when you want users to be able to customize the look and feel of their webpages. 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). Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Get help and advice from our experts on all things Burp. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. . Read more about DOM-based cross-site scripting. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. The third cross site scripting attack occurs entirely in the browser. 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. If you directly access an encoder via System.Text.Encodings.Web. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. More info about Internet Explorer and Microsoft Edge. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. *Encoder.Default then the default, Basic Latin only safelist will be used. Script manipulation: <script src> and setting text content of <script> elements. Tag helpers will also encode input you use in tag parameters. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. This could lead to an attack being added to a webpage.. for example. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. The logic which parses URLs in both execution and rendering contexts looks to be the same. If you're using JavaScript to change a CSS property, look into using style.property = x. 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. Encoding libraries often have a EncodeForJavaScript or similar to support this function. The DOM, or Document Object Model, is the structural format used to . DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Summary. The example that follows illustrates using closures to avoid double JavaScript encoding. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. 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. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. The primary difference is where the attack is injected into the application.

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