dom based cross site scripting prevention

Encode all characters using the \xHH format. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. This cheat sheet provides guidance to prevent XSS vulnerabilities. The name originated from early versions of the attack where stealing data cross-site was the primary focus. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. 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. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. 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. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. Always pass untrusted input as a query string value. Automatic encoding and escaping functions are built into most frameworks. The enterprise-enabled dynamic web vulnerability scanner. Sometimes you can't change the offending code. The web application dynamically generates a web page that contains this untrusted data. Validation can be a useful tool in limiting XSS attacks. One example of an attribute which is thought to be safe is innerText. 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. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Do your applications use this vulnerable package? OWASP recommends these in all circumstances. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. 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. This brings up an interesting design point. 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 (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. You need to work through each available source in turn, and test each one individually. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. 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. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. Don't use untrusted input as part of a URL path. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. 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. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. 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. Once you've found where the source is being read, you can use the JavaScript debugger to add a break point and follow how the source's value is used. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Read the entire Acunetix Web Application Vulnerability Report. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. 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). -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. 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. Never rely on validation alone. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. 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. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. Trusted Types force you to process a value. Otherwise, again, your security efforts are void. Don't mutate DOM directly. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. . Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. If you sanitize content and then modify it afterwards, you can easily void your security efforts. 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. 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. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". 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. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. DOM-based XSS is an advanced XSS attack. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. This means you will need to use alternative elements like img or iframe. 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. 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. This is a Safe Sink and will automatically CSS encode data in it. 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. The primary difference is where the attack is injected into the application. Before putting untrusted data into a URL query string ensure it's URL encoded. Download the latest version of Burp Suite. Sometimes users need to author HTML. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. DOM-based cross-site scripting attack DOM-based XSS is also sometimes called "type-0 XSS." It occurs when the XSS vector executes as a result of a DOM modification on a website in a user's browser. Document Object Model (DOM) Based XSS. Read more about DOM-based cross-site scripting. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. There are numerous methods which implicitly eval() data passed to it that must be avoided. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. These methods constitute the HTML Subcontext within the Execution Context. URL Contexts refer to variables placed into a URL. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. Semgrep rule to identify above dom xss link. Even newer versions of jQuery can still be vulnerable via the $() selector sink, provided you have full control over its input from a source that doesn't require a # prefix. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. The HTML encoded value above is still executable. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. DOM XSS stands for Document Object Model-based Cross-site Scripting. It uses HTML attribute encoding rules whenever you use the @ directive. This means, that no data will be available in server logs. If you're using JavaScript for writing to HTML, look at the .textContent attribute as it is a Safe Sink and will automatically HTML Entity Encode. 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. This helps quickly identify a large chunk of violations. Canonicalize input, URL Validation, Safe URL verification, Allow-list http and HTTPS URLs only (Avoid the JavaScript Protocol to Open a new Window), Attribute encoder. Acunetix developers and tech agents regularly contribute to the blog. Its critical to use quotation marks like " or ' to surround your variables. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. OWASP recommends DOMPurify for HTML Sanitization. Web Application Firewalls - These look for known attack strings and block them. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. The styling will not be rendered. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Now only JavaScript encoding on server side. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Therefore, the primary recommendation is to avoid including untrusted data in this context. Misconceptions abound related to the proper encoding that is required. Want to track your progress and have a more personalized learning experience? When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. 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. 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. For each location where your string appears within the DOM, you need to identify the context. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. For example, Acunetix. 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. The DOM is a programming interface. DOM Based Attacks. The #redir route is executed by another file, redir.html. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. //The following does NOT work because of the encoded ";". Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Please note, it is always dangerous design to put untrusted data directly into a command execution context. How to prevent DOM-based cross-site scripting? Read the entire Acunetix Web Application Vulnerability Report. 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. Some pure DOM-based vulnerabilities are self-contained within a single page. 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. 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. document.CreateTextNode () and append it in the appropriate DOM location. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. The only safe location for placing variables in JavaScript is inside a quoted data value. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. Please refer to the list below for details. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. There are a variety of sinks that are relevant to DOM-based vulnerabilities. There will be times where you need to do something outside the protection provided by your framework. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Here are some examples of encoded values for specific characters. In the case above, JavaScript encoding does not mitigate against DOM based XSS. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? This difference makes JavaScript encoding a less viable weapon in our fight against XSS. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. Get your questions answered in the User Forum. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). Scale dynamic scanning. There are two distinct groups of cross-site scripting. 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. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. The name originated from early versions of the attack where stealing data cross-site was the primary focus. In some . For that, first create a policy. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. Learn the details here including XSS prevention methods. There are 3 primary types of cross-site scripting: DOM-based XSS. 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. Encoding libraries often have a EncodeForJavaScript or similar to support this function. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. 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. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. There are two ways to do this. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. No single technique will solve XSS. A Computer Science portal for geeks. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. 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. Each encoder, Html, JavaScript and Url, must be configured separately. Cross-Site Scripting (XSS) is a misnomer. Trusted Types require you to process the data before passing it to the above sink functions. The best manual tools to start web security testing. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). 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. DOM-based XSS is a kind of XSS occurring entirely on the client-side. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. //The following does NOT work because of the encoded "(" and ")". The following is an example vulnerability which occurs in the JavaScript context and HTML subcontext: Let's look at the individual subcontexts of the execution context in turn. 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 site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. You might find that the source gets assigned to other variables. If you must, the following examples describe some approaches that do and do not work. 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. However the opposite is the case with HTML encoding. 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. 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=. Free, lightweight web application security scanning for CI/CD. Finally, to fix the problem in our initial code, instead of trying to encode the output correctly which is a hassle and can easily go wrong we would simply use element.textContent to write it in a content like this: It does the same thing but this time it is not vulnerable to DOM based cross-site scripting vulnerabilities. If your code looked like the following, you would need to only double JavaScript encode input data. The other alternative is using N-levels of encoding. CSS is surprisingly powerful and has been used for many types of attacks. your framework), you should be able to mitigate all XSS vulnerabilities. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. There are a couple of options for fixing a Trusted Type violation. Testing JavaScript execution sinks for DOM-based XSS is a little harder. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. The example that follows illustrates using closures to avoid double JavaScript encoding. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. HTML tag elements are well defined and do not support alternate representations of the same tag. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. In DOM-based cross-site scripting, the HTML source code and response of the attack . Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. The line above could have possibly worked to render a link. Use only safe functions like document.innerText and document.textContent. 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. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. //The following does NOT work because the event handler is being set to a string. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. 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). Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. How to detect DOM-based cross-site scripting? The data is subsequently read from the DOM by the web application and outputted to the browser. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. //The following DOES WORK because the encoded value is a valid variable name or function reference. 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. XSS Prevention & Mitigation. Script manipulation: <script src> and setting text content of <script> elements. 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.

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