Never rely on validation alone. Semgrep rule to identify above dom xss link. Event handlers such as onload and onerror can be used in conjunction with these elements. 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. DOM-based XSS Examples. Trusted Types require you to process the data before passing it to the above sink functions. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. 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. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Output encoding here will prevent XSS, but it will break the intended functionality of the application. In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. For instance, jQuery's attr() function can change the attributes of DOM elements. In this section, we'll describe DOM-based cross-site scripting (DOM XSS), explain how to find DOM XSS vulnerabilities, and talk about how to exploit DOM XSS with different sources and sinks. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. If your code looked like the following, you would need to only double JavaScript encode input data. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. DOM-Based Cross-Site Scripting. 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 ">". In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. Websites may also store data on the server and reflect it elsewhere. URL Contexts refer to variables placed into a URL. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Sometimes you can't change the offending code. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. 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. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. your framework), you should be able to mitigate all XSS vulnerabilities. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. What's the difference between Pro and Enterprise Edition? Level up your hacking and earn more bug bounties. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? For DOM XSS, the attack is injected into the application during runtime in the client directly. 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. 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. 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 example, you might need to close some existing elements before using your JavaScript payload. We want to hear from you! The styling will not be rendered. Get your questions answered in the User Forum. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Other CSS Contexts are unsafe and you should not place variable data in them. The DOM is a programming interface. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. Use only safe functions like document.innerText and document.textContent. 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. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. This is a Safe Sink and will automatically CSS encode data in it. Free, lightweight web application security scanning for CI/CD. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. 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. Catch critical bugs; ship more secure software, more quickly. 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. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. Scale dynamic scanning. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. 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. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Limit access to object properties when using object[x] accessors (Mike Samuel). In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). 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. If you sanitize content and then modify it afterwards, you can easily void your security efforts. If you're using JavaScript to change a CSS property, look into using style.property = x. Stored XSS is considered the most damaging type of XSS attack. Output encoding is not perfect. How to prevent DOM-based cross-site scripting? Don't use untrusted input as part of a URL path. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. What would be displayed in the input text field would be "Johnson & Johnson". Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. You need to work through each available source in turn, and test each one individually. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. This is common when you want users to be able to customize the look and feel of their webpages. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. . To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. //The following does NOT work because of the encoded "(" and ")". Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). 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. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. On the client side, the HTTP response does not change but the script executes in malicious manner. DOM XSS stands for Document Object Model-based Cross-site Scripting. It will not always prevent XSS. Based on this context, you need to refine your input to see how it is processed. It is an informational message with a simple alert. Any application is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can develop from source to sink. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. 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. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. Do your applications use this vulnerable package? \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');",