There are numerous methods which implicitly eval() data passed to it that must be avoided. In JavaScript code, the main context is JavaScript but with the right tags and context closing characters, an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods. 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. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. What is Cross-Site Scripting (XSS) and How to Prevent It? WAFs are not recommended for preventing XSS, especially DOM-Based XSS. What is WordPress Cross-site Scripting (XSS) and How to prevent it? In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. Get started with Burp Suite Professional. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. (It's free!). Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. More info about Internet Explorer and Microsoft Edge. XSS is one of the most common and dangerous web vulnerabilities, and it is . 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. Catch critical bugs; ship more secure software, more quickly. Thankfully, many sinks where variables can be placed are safe. Output Encoding and HTML Sanitization help address those gaps. 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. Output encoding is the primary defense against cross-site scripting vulnerabilities. We will look at eval, href and dangerouslySetHTML vulnerabilities. The HTML parser of the rendering context dictates how data is presented and laid out on the page and can be further broken down into the standard contexts of HTML, HTML attribute, URL, and CSS. 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. XSS sinks are places where variables are placed into your webpage. DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks How to prevent cross-site scripting attacks | Infosec Resources The data is subsequently read from the DOM by the web application and outputted to the browser. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. HTML tag elements are well defined and do not support alternate representations of the same tag. //any code passed into lName is now executable. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. Read more about DOM-based cross-site scripting. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. 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. 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. If you're using JavaScript to change a CSS property, look into using style.property = x. Safe list ranges are specified as Unicode code charts, not languages. Customization of the safe list only affects encoders sourced via DI. The rendered output would now become. 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. DOM based XSS vulnerabilities therefore have to be prevented on the client side. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). You might find that the source gets assigned to other variables. This means you will need to use alternative elements like img or iframe. 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. A Computer Science portal for geeks. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Here are some examples of encoded values for specific characters. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. 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. Also, XSS attacks always execute in the browser. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. This is where Output Encoding and HTML Sanitization are critical. 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. All the Acunetix developers come with years of experience in the web security sphere. Learn more about types of cross-site scripting attacks In the case above, JavaScript encoding does not mitigate against DOM based XSS. For a comprehensive list, check out the DOMPurify allowlist. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. For example if you want to use user input to write in a div tag element don't use innerHtml, instead use innerText or textContent. The world's #1 web penetration testing toolkit. Understanding the XSS Threat: A Comprehensive Guide to DOM Based Cross I will show you three examples of DOM-based XSS attacks in this article. DOM XSS stands for Document Object Model-based Cross-site Scripting. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. A Complete Guide To Cross Site Scripting - fas3c7.blogspot.com 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. //The following does NOT work because of the encoded "(" and ")". This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. 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 is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. 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. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. 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. For example, if your string appears within a double-quoted attribute then try to inject double quotes in your string to see if you can break out of the attribute. It is the process of converting untrusted . 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. CSS Contexts refer to variables placed into inline CSS. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. 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 ">". Encoding libraries often have a EncodeForJavaScript or similar to support this function. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. 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. That said, developers need to be aware of problems that can occur when using frameworks insecurely such as: Understand how your framework prevents XSS and where it has gaps. The best manual tools to start web security testing. This can be done via a function such as: What is XSS? Impact, Types, and Prevention - Bright Security Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Get the latest content on web security in your inbox each week. Limit access to object properties when using object[x] accessors (Mike Samuel). In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. Level up your hacking and earn more bug bounties. 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. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Browsers change functionality and bypasses are being discovered regularly. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable 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=. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. The only safe location for placing variables in JavaScript is inside a quoted data value. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. Other CSS Contexts are unsafe and you should not place variable data in them. XSS Prevention & Mitigation. 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. What is Cross-Site Scripting (XSS)? How to Prevent it? | Fortinet While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. This is because these sinks treat the variable as text and will never execute it. Trusted Types force you to process a value. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. OWASP recommends DOMPurify for HTML Sanitization. How common is DOM-based cross-site scripting? This is a Safe Sink and will automatically CSS encode data in it. DOM Based Attacks. //The following DOES WORK because the encoded value is a valid variable name or function reference. It is an informational message with a simple alert. 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. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. Before putting untrusted data inside an HTML element ensure it's HTML encoded. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. 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. javascript - XSS prevention and .innerHTML - Stack Overflow Free, lightweight web application security scanning for CI/CD. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Script manipulation: <script src> and setting text content of <script> elements. Before putting untrusted data into a URL query string ensure it's URL encoded. Sometimes you can't change the offending code. For example. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. \u0064\u006f\u0063\u0075\u006d\u0065\u006e\u0074, \u0077\u0072\u0069\u0074\u0065\u006c\u006e, "\u0048\u0065\u006c\u006c\u006f\u0020\u0057\u006f\u0072\u006c\u0064", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0031\u0029", "url(<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(companyName))%>)", '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(userRelativePath))%>', "<%= Encode.forJavaScript(untrustedData) %>", "<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>", "customFunction('<%=doubleJavaScriptEncodedData%>', y)", //HTML encoding is happening in JavaScript, "javascript:myFunction('<%=untrustedData%>', 'test');", "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(untrustedData)) %>', 'test');",