You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. Free, lightweight web application security scanning for CI/CD. 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. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. For DOM XSS, the attack is injected into the application during runtime in the client directly. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Otherwise, again, your security efforts are void. It will not always prevent XSS. Learn more about types of cross-site scripting attacks Catch critical bugs; ship more secure software, more quickly. How to prevent DOM-based cross-site scripting? The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. 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. Use a CSP as an additional layer of defense and have a look at the. Each variable in a web application needs to be protected. Always pass untrusted input as a query string value. Common injection vectors include document.url, document.location, and document.referrer objects. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. 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. 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 a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. This helps quickly identify a large chunk of violations. Its critical to use quotation marks like " or ' to surround your variables. 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. Output encoding here will prevent XSS, but it will break the intended functionality of the application. This could lead to an attack being added to a webpage.. for example. This brings up an interesting design point. Get started with Burp Suite Professional. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Consider adopting the following controls in addition to the above. Script manipulation: <script src> and setting text content of <script> elements. 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. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. For example, websites often reflect URL parameters in the HTML response from the server. For example, Acunetix. Accelerate penetration testing - find more bugs, more quickly. There will be situations where you use a URL in different contexts. In other words, add a level of indirection between untrusted input and specified object properties. Always encode untrusted input before output, no matter what validation or sanitization has been performed. 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 you must, the following examples describe some approaches that do and do not work. It is the process of converting untrusted . Parsing HTML input is difficult, if not impossible. //The following DOES WORK because the encoded value is a valid variable name or function reference. 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. 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. One example of an attribute which is thought to be safe is innerText. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). For example, you might need to close some existing elements before using your JavaScript payload. 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. DOM-based XSS: DOM-based XSS occurs when an . Acunetix developers and tech agents regularly contribute to the blog. 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. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. However, depending on the tag which innerText is applied, code can be executed. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. The logic which parses URLs in both execution and rendering contexts looks to be the same. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. 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. No single technique will solve XSS. This means, that no data will be available in server logs. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. These methods constitute the HTML Subcontext within the Execution Context. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. This is because these sinks treat the variable as text and will never execute it. Input validation. 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()). Using the right combination of defensive techniques is necessary to prevent XSS. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. 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. This cheat sheet provides guidance to prevent XSS vulnerabilities. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. // is an example of untrusted data that was properly JavaScript encoded but still executes. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. 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. It is a simple yet effective way to harvest passwords using only the victims browser. Practise exploiting vulnerabilities on realistic targets. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. You need to work through each available source in turn, and test each one individually. This is why you would need to HTML encode too. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. *Encoder.Default then the default, Basic Latin only safelist will be used. Sometimes you can't change the offending code. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. 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. 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(). If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Some examples of DOM-based XSS attacks include: 1. If you pollute a river, it'll flow downstream somewhere. Misconceptions abound related to the proper encoding that is required. 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. There are two distinct groups of cross-site scripting. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Level up your hacking and earn more bug bounties. There are numerous methods which implicitly eval() data passed to it that must be avoided. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. The problem is that if companyName had the value "Johnson & Johnson". This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. HTML tag elements are well defined and do not support alternate representations of the same tag. Get your questions answered in the User Forum. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. Never rely on validation alone. Avoid populating the following methods with untrusted data. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. In the case above, JavaScript encoding does not mitigate against DOM based XSS. If this is the case, you'll need to use the search function again to track these variables and see if they're passed to a sink. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. 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. For a comprehensive list, check out the DOMPurify allowlist. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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 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. 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. Content Security Policy - An allowlist that prevents content being loaded. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. Reduce the DOM XSS attack surface of your application. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. It's important to remember that some of these are also potential sources and sinks for DOM XSS. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. So XSS has already been around for a while. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. If these methods are provided with untrusted input, then an XSS vulnerability could result. Don't use untrusted input as part of a URL path. 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 XSS stands for Document Object Model-based Cross-site Scripting. Already got an account? 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. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. There are a variety of sinks that are relevant to DOM-based vulnerabilities. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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 XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. The DOM is a programming interface. There will be times where you need to do something outside the protection provided by your framework. Save time/money. If A is double JavaScript encoded then the following if check will return false. The DOM-based cross-site scripting requires the user to open an infected page. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. 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. 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. \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');",