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

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. The styling will not be rendered. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. OWASP recommends DOMPurify for HTML Sanitization. The other alternative is using N-levels of encoding. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. The DOM is a programming interface. Validation can be a useful tool in limiting XSS attacks. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Some examples of DOM-based XSS attacks include: 1. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. In a DOM-based attacks, the HTTP response on the server side does not change. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. For example, websites often reflect URL parameters in the HTML response from the server. WAFs also dont address the root cause of an XSS vulnerability. A list of output encoding libraries is included in the appendix. An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! Any variable that does not go through this process is a potential weakness. 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). It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Validation becomes more complicated when accepting HTML in user input. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. All other contexts are unsafe and you should not place variable data in them. (It's free!). In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. 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. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. 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. For instance, jQuery's attr() function can change the attributes of DOM elements. Output encoding is the primary defense against cross-site scripting vulnerabilities. Some pure DOM-based vulnerabilities are self-contained within a single page. Use URL Encoding for these scenarios. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. Cross-Site Scripting (XSS) is a misnomer. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Developers should use the following prevention steps to avoid introducing XSS into their application. 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. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. If your code looked like the following, you would need to only double JavaScript encode input data. 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. 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. Learn the details here including XSS prevention methods. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. Fewer XSS bugs appear in applications built with modern web frameworks. . It will not always prevent XSS. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. Trusted Types heavily reduce the DOM XSS attack surface of your application. Using the right combination of defensive techniques is necessary to prevent XSS. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. . //The following does NOT work because the event handler is being set to a string. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Summary. 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. Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn't properly filter user input. This variable includes some characters which are used in XSS attacks, namely <, " and >. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. What's the difference between Pro and Enterprise Edition? These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Each encoder, Html, JavaScript and Url, must be configured separately. This fact makes it more difficult to maintain web application security. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. However the opposite is the case with HTML encoding. The Unicode standard has a list of code charts you can use to find the chart containing your characters. 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. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Otherwise, again, your security efforts are void. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. *Encoder.Default then the default, Basic Latin only safelist will be used. Read more about DOM-based cross-site scripting. 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). To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. It is an informational message with a simple alert. Websites may also store data on the server and reflect it elsewhere. The only safe location for placing variables in JavaScript is inside a quoted data value. The HTML encoded value above is still executable. Variables should not be interpreted as code instead of text. For example. Get started with Burp Suite Professional. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. 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. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. Please refer to the list below for details. 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. This is because these sinks treat the variable as text and will never execute it. Stored XSS is considered the most damaging type of XSS attack. Use a trusted and verified library to escape HTML inputs. Use a CSP as an additional layer of defense and have a look at the. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. //any code passed into lName is now executable. Read about other types of cross-site scripting attacks. . The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. This is why you would need to HTML encode too. document.CreateTextNode () and append it in the appropriate DOM location. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. 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. Despite being rare, they may cause serious problems and only a few scanners can detect them. If you directly access an encoder via System.Text.Encodings.Web. 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. It is also impossible to protect against such client-side attacks using WAFs. This can be done via a function such as: In these scenarios, you should do URL encoding, followed by HTML attribute encoding. 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. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Enhance security monitoring to comply with confidence. This cheat sheet provides guidance to prevent XSS vulnerabilities. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. Event handlers such as onload and onerror can be used in conjunction with these elements. placed in an HTML Attribute. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. //The following DOES WORK because the encoded value is a valid variable name or function reference. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. There will be situations where you use a URL in different contexts. Get help and advice from our experts on all things Burp. 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. 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). The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. See how our software enables the world to secure the web. 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. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. Based on this context, you need to refine your input to see how it is processed. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. DOM XSS stands for Document Object Model-based Cross-site Scripting. All the Acunetix developers come with years of experience in the web security sphere. 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=. The third cross site scripting attack occurs entirely in the browser. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. eval Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. 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. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. The best way to fix DOM based cross-site scripting is to use the right output method (sink). HTML tag elements are well defined and do not support alternate representations of the same tag. Encode all characters using the \xHH format. 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 ">". Then client-side encode (using a JavaScript encoding library such as node-esapi) for the individual subcontext (DOM methods) which untrusted data is passed to. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. See what Acunetix Premium can do for you. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. The 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. Want to track your progress and have a more personalized learning experience? This means you will need to use alternative elements like img or iframe. This helps quickly identify a large chunk of violations. Here are some examples of encoded values for specific characters. Thankfully, many sinks where variables can be placed are safe. Document Object Model (DOM) Based XSS. Avoid populating the following methods with untrusted data. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. There are some further things to consider: Security professionals often talk in terms of sources and sinks. 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. The logic which parses URLs in both execution and rendering contexts looks to be the same. However, you may still find vulnerable code in the wild. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. If you're using JavaScript to change a CSS property, look into using style.property = x. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets.