This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. What is cross-site scripting (XSS) and how to prevent it? | Web Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. Before putting untrusted data into a URL query string ensure it's URL encoded. Read more about DOM-based cross-site scripting. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Cross-site scripting XSS To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. There are 3 primary types of cross-site scripting: DOM-based XSS. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. 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. Stored XSS is considered the most damaging type of XSS attack. 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. Each variable used in the user interface should be passed through an output encoding function. A script within the later response contains a sink which then processes the data in an unsafe way. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. DOM XSS: An Explanation of DOM-based Cross-site Scripting 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. Record your progression from Apprentice to Expert. The #redir route is executed by another file, redir.html. For each potential source, such as location, you first need to find cases within the page's JavaScript code where the source is being referenced. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. The Unicode standard has a list of code charts you can use to find the chart containing your characters. This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against XSS. Scale dynamic scanning. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. The primary difference is where the attack is injected into the application. Cross-Site Scripting (XSS) is a misnomer. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. The third cross site scripting attack occurs entirely in the browser. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. Developers should use the following prevention steps to avoid introducing XSS into their application. Despite being rare, they may cause serious problems and only a few scanners can detect them. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. 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. Using untrusted user data on the left side of the expression allows an attacker to subvert internal and external attributes of the window object, whereas using user input on the right side of the expression doesn't allow direct manipulation. DOM based cross site scripting (Video solution) - YouTube What is Cross-Site Scripting (XSS)? Definition and Prevention - Rapid7 If you pollute a river, it'll flow downstream somewhere. We are looking for web developers to participate in user research, product testing, discussion groups and more. However, you may still find vulnerable code in the wild. How to Prevent Cross Site Scripting | XSS Attack Prevention These locations are known as dangerous contexts. Please note, element.setAttribute is only safe for a limited number of attributes. 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. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. Catch critical bugs; ship more secure software, more quickly. DOM-based XSS Examples. All the Acunetix developers come with years of experience in the web security sphere. Always pass untrusted input as a query string value. It's important to remember that some of these are also potential sources and sinks for DOM XSS. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. For details, see the Google Developers Site Policies. On the client side, the HTTP response does not change but the script executes in malicious manner. Get your questions answered in the User Forum. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Make sure any attributes are fully quoted, same as JS and CSS. Use URL Encoding for these scenarios. WAFs also dont address the root cause of an XSS vulnerability. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. What is DOM-based XSS (cross-site scripting)? - Invicti . This document only discusses JavaScript bugs which lead to XSS. Safe list ranges are specified as Unicode code charts, not languages. 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. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. Policies are factories for Trusted Types that enforce certain security rules on their input: This code creates a policy called myEscapePolicy that can produce TrustedHTML objects via its createHTML() function. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. Variables should only be placed in a CSS property value. DOM-Based Cross-Site Scripting (DOM XSS) | Learn AppSec - Invicti When your application no longer produces violations, you can start enforcing Trusted Types: Voila! 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. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. If A is double JavaScript encoded then the following if check will return false. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. For example, this is the case if you're loading a third-party library from a CDN. So XSS has already been around for a while. Encode all characters using the \xHH format. Free, lightweight web application security scanning for CI/CD. 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. DOM based XSS vulnerabilities therefore have to be prevented on the client side. //any code passed into lName is now executable. DOM based XSS Prevention - OWASP Cheat Sheet Series This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). . Use only safe functions like document.innerText and document.textContent. 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. 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. 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). Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. 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. Consider adopting the following controls in addition to the above. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Common injection vectors include document.url, document.location, and document.referrer objects. What's the best way to prevent XSS attacks? | TechTarget It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Based on this context, you need to refine your input to see how it is processed. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Login here. OWASP recommends these in all circumstances. 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. Browsers change functionality and bypasses are being discovered regularly. Information on ordering, pricing, and more. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. 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. element.SetAttribute () element [attribute]= When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. These types of attacks typically occur as a result . If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Avoid populating the following methods with untrusted data. A Computer Science portal for geeks. DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Safe HTML Attributes include: 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. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Encoding libraries often have a EncodeForJavaScript or similar to support this function. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. HTML Context refers to inserting a variable between two basic HTML tags like a or
. In a DOM-based attacks, the HTTP response on the server side does not change. 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. 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 best manual tools to start web security testing. 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. React XSS Cross-site scripting prevention - Dev Academy The DOM, or Document Object Model, is the structural format used to . 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. 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. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. Use a trusted and verified library to escape HTML inputs. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. The DOM is a programming interface. 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). 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. Get the latest content on web security in your inbox each week. 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. 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. See what Acunetix Premium can do for you. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. javascript - XSS prevention and .innerHTML - Stack Overflow The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. DOM based XSS Prevention Cheat Sheet - GitHub As we use reCAPTCHA, you need to be able to access Google's servers to use this function. You might find that the source gets assigned to other variables. DOM Based Attacks. 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. All of this code originates on the server, which means it is the application owner's responsibility to make it safe from XSS, regardless of the type of XSS flaw it is. These methods constitute the HTML Subcontext within the Execution Context. 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. It is a simple yet effective way to harvest passwords using only the victims browser. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. The HTML encoded value above is still executable. 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. 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. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. After the page's JavaScript applies this malicious URL to the back link's href, clicking on the back link will execute it: Another potential sink to look out for is jQuery's $() selector function, which can be used to inject malicious objects into the DOM. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Accelerate penetration testing - find more bugs, more quickly. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. The world's #1 web penetration testing toolkit. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. In some . DOM-based XSS is a kind of XSS occurring entirely on the client-side. Read more about DOM-based cross-site scripting. The most common one would be adding it to an href or src attribute of an tag. 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). 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. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. There are a variety of sinks that are relevant to DOM-based vulnerabilities. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). Practise exploiting vulnerabilities on realistic targets. XSS sinks are places where variables are placed into your webpage. 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. In other words, add a level of indirection between untrusted input and specified object properties. No single technique will solve XSS. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. 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. 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. If you're using JavaScript to change a CSS property, look into using style.property = x. CSS Contexts refer to variables placed into inline CSS. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. What is Cross-Site Scripting (XSS) and How to Prevent It? The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. 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 enterprise-enabled dynamic web vulnerability scanner. For that, first create a policy. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. This is a Safe Sink and will automatically CSS encode data in it.