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. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. 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. DOM-based XSS Examples. (It's free!). Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. 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. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Event handlers such as onload and onerror can be used in conjunction with these elements. DOM XSS stands for Document Object Model-based Cross-site Scripting. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. . Get help and advice from our experts on all things Burp. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. Common injection vectors include document.url, document.location, and document.referrer objects. This is common when you want users to be able to customize the look and feel of their webpages. 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. 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. 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. If your code looked like the following, you would need to only double JavaScript encode input data. The web application dynamically generates a web page that contains this untrusted data. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. WAFs also dont address the root cause of an XSS vulnerability. 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. Normally executing JavaScript from a CSS context required either passing javascript:attackCode() to the CSS url() method or invoking the CSS expression() method passing JavaScript code to be directly executed. The HTML encoded value above is still executable. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. 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. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. In DOM-based cross-site scripting, the HTML source code and response of the attack . DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. Variables should only be placed in a CSS property value. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. How to detect DOM-based cross-site scripting? In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. Read the entire Acunetix Web Application Vulnerability Report. The reason why you only need to double JavaScript encode is that the customFunction function did not itself pass the input to another method which implicitly or explicitly called eval If firstName was passed to another JavaScript method which implicitly or explicitly called eval() then <%=doubleJavaScriptEncodedData%> above would need to be changed to <%=tripleJavaScriptEncodedData%>. 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. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. For each location where your string appears within the DOM, you need to identify the context. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. For example. 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 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. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. It is a simple yet effective way to harvest passwords using only the victims browser. element.SetAttribute () element [attribute]= Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. 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. 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. As with all other Cross-site Scripting (XSS) vulnerabilities, this type of attack also relies on insecure handling of user input on an HTML page. //any code passed into lName is now executable. The third cross site scripting attack occurs entirely in the browser. Misconceptions abound related to the proper encoding that is required. Ideally, the correct way to apply encoding and avoid the problem stated above is to server-side encode for the output context where data is introduced into the application. 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. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). 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. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. 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. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. 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. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. Login here. WAFs are unreliable and new bypass techniques are being discovered regularly. 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). Accelerate penetration testing - find more bugs, more quickly. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. It is also impossible to protect against such client-side attacks using WAFs. 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. 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). Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. The DOM-based cross-site scripting requires the user to open an infected page. 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. All the Acunetix developers come with years of experience in the web security sphere. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Other CSS Contexts are unsafe and you should not place variable data in them. These methods constitute the HTML Subcontext within the Execution Context. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. This brings up an interesting design point. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. It is the process of converting untrusted . // is an example of untrusted data that was properly JavaScript encoded but still executes. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. Before putting untrusted data inside an HTML element ensure it's HTML encoded. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. 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. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. I will show you three examples of DOM-based XSS attacks in this article. 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. HTML tag elements are well defined and do not support alternate representations of the same tag. 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. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. 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. 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). //The following DOES WORK because the encoded value is a valid variable name or function reference. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. Get your questions answered in the User Forum. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. If you directly access an encoder via System.Text.Encodings.Web. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. Encode all characters with the %HH encoding format. One example of an attribute which is thought to be safe is innerText. 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. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". Sometimes you can't change the offending code. If you're using JavaScript to change a CSS property, look into using style.property = x. There are two distinct groups of cross-site scripting. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. 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. 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. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. What's the difference between Pro and Enterprise Edition? You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. In those cases, create a Trusted Type object yourself. Fewer XSS bugs appear in applications built with modern web frameworks. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. 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. Trusted Types require you to process the data before passing it to the above sink functions. Despite being rare, they may cause serious problems and only a few scanners can detect them. This is why you would need to HTML encode too. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. 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. Encoding libraries often have a EncodeForJavaScript or similar to support this function.
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