dom based cross site scripting prevention

It uses HTML attribute encoding rules whenever you use the @ directive. 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. 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 most common source for DOM XSS is the URL, which is typically accessed with the window.location object. 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. This is common when you want users to be able to customize the look and feel of their webpages. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. There are two distinct groups of cross-site scripting. Don't mutate DOM directly. For each location where your string appears within the DOM, you need to identify the context. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. 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. 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. Learn the details here including XSS prevention methods. 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. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Output encoding is not perfect. Customization of the safe list only affects encoders sourced via DI. Information on ordering, pricing, and more. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. 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. 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. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. More info about Internet Explorer and Microsoft Edge. DOM-based XSS: In this type of attack, the attacker injects malicious code into a web page that is executed on the client-side within the Document Object Model (DOM) of the web page. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. CSS Contexts refer to variables placed into inline CSS. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. Limit access to object properties when using object[x] accessors (Mike Samuel). In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Also, XSS attacks always execute in the browser. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Avoid populating the following methods with untrusted data. There are some further things to consider: Security professionals often talk in terms of sources and sinks. WAFs also dont address the root cause of an XSS vulnerability. How to detect DOM-based cross-site scripting? If these methods are provided with untrusted input, then an XSS vulnerability could result. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. What's the difference between Pro and Enterprise Edition? It is the process of converting untrusted . . Cross-Site Scripting (XSS) is a misnomer. 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. Get help and advice from our experts on all things Burp. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. 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). Stored XSS is considered the most damaging type of XSS attack. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. . The HTML encoded value above is still executable. Download the latest version of Burp Suite. Websites may also store data on the server and reflect it elsewhere. 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. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Developers should use the following prevention steps to avoid introducing XSS into their application. . For that, first create a policy. DOM-based XSS is an advanced XSS attack. Sometimes you can't change the offending code. Web Application Firewalls - These look for known attack strings and block them. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Consider adopting the following controls in addition to the above. The only safe location for placing variables in JavaScript is inside a quoted data value. Already got an account? This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. There are numerous methods which implicitly eval() data passed to it that must be avoided. It is an informational message with a simple alert. 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. Save time/money. How to prevent DOM-based cross-site scripting? In many cases the context isn't always straightforward to discern. Thankfully, many sinks where variables can be placed are safe. 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Do your applications use this vulnerable package? 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. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . 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. 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. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. Testing JavaScript execution sinks for DOM-based XSS is a little harder. 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 most common one would be adding it to an href or src attribute of an tag. A rendering context is associated with the parsing of HTML tags and their attributes. For example, when your application passes a string to innerHTML, the browser sends the following report: This says that in https://my.url.example/script.js on line 39 innerHTML was called with the string beginning with <img src=x. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. 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. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. Validate all data that flows into your application from the server or a third-party API. The best way to fix DOM based cross-site scripting is to use the right output method (sink). The best way to fix DOM based cross-site scripting is to use the right output method (sink). Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. 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 ">". This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. For a comprehensive list, check out the DOMPurify allowlist. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Therefore, the primary recommendation is to avoid including untrusted data in this context. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. What would be displayed in the input text field would be "Johnson & Johnson". It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. This is a Safe Sink and will automatically URL encode data in it. For instance, jQuery's attr() function can change the attributes of DOM elements. There will be situations where you use a URL in different contexts. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. We want to hear from you! The best manual tools to start web security testing. You need to work through each available source in turn, and test each one individually. If you sanitize content and then modify it afterwards, you can easily void your security efforts. Safe list ranges are specified as Unicode code charts, not languages. 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. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. WAFs are unreliable and new bypass techniques are being discovered regularly. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). In DOM-based cross-site scripting, the HTML source code and response of the attack . 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. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. In the case above, JavaScript encoding does not mitigate against DOM based XSS. HTML tag elements are well defined and do not support alternate representations of the same tag. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Get started with Burp Suite Enterprise Edition. Tag helpers will also encode input you use in tag parameters. OWASP recommends DOMPurify for HTML Sanitization. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. 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. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. 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. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. The line above could have possibly worked to render a link. Please note, it is always dangerous design to put untrusted data directly into a command execution context. It is a simple yet effective way to harvest passwords using only the victims browser. 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. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. Its the same with computer security. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. Now only JavaScript encoding on server side. 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. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. 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. Accelerate penetration testing - find more bugs, more quickly. For DOM XSS, the attack is injected into the application during runtime in the client directly. Fewer XSS bugs appear in applications built with modern web frameworks. In those cases, create a Trusted Type object yourself. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. The DOM-based cross-site scripting requires the user to open an infected page. A list of safe HTML attributes is provided in the Safe Sinks section. 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. 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. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Output encoding is the primary defense against cross-site scripting vulnerabilities. For example.. An attacker could modify data that is rendered as $varUnsafe. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Despite being rare, they may cause serious problems and only a few scanners can detect them. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. On the client side, the HTTP response does not change but the script executes in malicious manner. If this isn't possible, then ensure the data is JavaScript encoded. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. The DOM, or Document Object Model, is the structural format used to . The good news is that if user input is handled properly at the foundation level (e.g. Cross-Site Scripting (XSS) is a misnomer. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. A script within the later response contains a sink which then processes the data in an unsafe way. This cheat sheet provides guidance to prevent XSS vulnerabilities. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. 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. Examples of safe attributes includes: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. 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. Read more about DOM-based cross-site scripting. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. 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. 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(). This can be done via a function such as: 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. This could lead to an attack being added to a webpage.. for example. 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. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Based on this context, you need to refine your input to see how it is processed. 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. DOM-based XSS: DOM-based XSS occurs when an . 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. 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. Variables should only be placed in a CSS property value. HTML Context refers to inserting a variable between two basic HTML tags like a