This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. 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. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn't properly filter user input. You might find that the source gets assigned to other variables. Therefore, the primary recommendation is to avoid including untrusted data in this context. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. 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. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. 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. 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. 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. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) What would be displayed in the input text field would be "Johnson & Johnson". It is a simple yet effective way to harvest passwords using only the victims browser. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. 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. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. 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. In these cases, HTML Sanitization should be used. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. It uses HTML attribute encoding rules whenever you use the @ directive. This is because these sinks treat the variable as text and will never execute it. Cross-Site Scripting (XSS) is a misnomer. More info about Internet Explorer and Microsoft Edge. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Validation becomes more complicated when accepting HTML in user input. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Trusted Types force you to process a value. We are looking for web developers to participate in user research, product testing, discussion groups and more. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Summary. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). This should never be used in combination with untrusted input as this will expose an XSS vulnerability. DOMPurify supports Trusted Types and will return sanitized HTML wrapped in a TrustedHTML object such that the browser does not generate a violation.CautionIf the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. Sometimes users need to author HTML. placed in an HTML Attribute. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. 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. WAFs are unreliable and new bypass techniques are being discovered regularly. In DOM-based cross-site scripting, the HTML source code and response of the attack . This can be done via a function such as: Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! //The following DOES WORK because the encoded value is a valid variable name or function reference. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). When this happens, a script on the web page selects the URL variable and executes the code it contains. 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(). It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Already got an account? For example, you might need to close some existing elements before using your JavaScript payload. 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. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Scale dynamic scanning. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. Limit access to object properties when using object[x] accessors (Mike Samuel). The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. The best way to fix DOM based cross-site scripting is to use the right output method (sink). To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. In many cases the context isn't always straightforward to discern. Make sure any attributes are fully quoted, same as JS and CSS. Use only safe functions like document.innerText and document.textContent. 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. Get started with Burp Suite Enterprise Edition. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. The HTML encoded value above is still executable. CSS is surprisingly powerful and has been used for many types of attacks. 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. There may be times you want to insert a value into JavaScript to process in your view. 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. There are several methods and attributes which can be used to directly render HTML content within JavaScript. If you directly access an encoder via System.Text.Encodings.Web. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. Output encoding is not perfect. Information on ordering, pricing, and more. Read more about DOM-based cross-site scripting. We will look at eval, href and dangerouslySetHTML vulnerabilities. Otherwise, again, your security efforts are void. The third cross site scripting attack occurs entirely in the browser. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. your framework), you should be able to mitigate all XSS vulnerabilities. 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. 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. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. Validation can be a useful tool in limiting XSS attacks. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. Some examples of DOM-based XSS attacks include: 1. 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. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. 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. Stored XSS is considered the most damaging type of XSS attack. DOM-based XSS is a kind of XSS occurring entirely on the client-side. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. What's the difference between Pro and Enterprise Edition? Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? 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. Free, lightweight web application security scanning for CI/CD. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. If this isn't possible, then ensure the data is JavaScript encoded. The DOM-based cross-site scripting requires the user to open an infected page. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. 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. The example that follows illustrates using closures to avoid double JavaScript encoding. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. XSS Prevention & Mitigation. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. The other alternative is using N-levels of encoding. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. document.CreateTextNode () and append it in the appropriate DOM location. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. How to prevent DOM-based cross-site scripting? 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. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. OWASP recommends these in all circumstances. Validate all data that flows into your application from the server or a third-party API. Document Object Model (DOM) Based XSS. This brings up an interesting design point. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. This is why you would need to HTML encode too. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. 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. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. The enterprise-enabled dynamic web vulnerability scanner. HTML Context refers to inserting a variable between two basic HTML tags like a
or . 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. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. Before putting untrusted data into a URL query string ensure it's URL encoded. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. There will be times where you need to do something outside the protection provided by your framework. 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. So XSS has already been around for a while. 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. DOM Based Attacks. 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. 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. For a comprehensive list, check out the DOMPurify allowlist. Output encoding is the primary defense against cross-site scripting vulnerabilities. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. 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. //any code passed into lName is now executable. If these methods are provided with untrusted input, then an XSS vulnerability could result. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. Any variable that does not go through this process is a potential weakness. 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. This variable includes some characters which are used in XSS attacks, namely <, " and >. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. Use a CSP as an additional layer of defense and have a look at the. The line above could have possibly worked to render a link. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. Other CSS Contexts are unsafe and you should not place variable data in them. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. 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. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. For that, first create a policy. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. Never rely on validation alone. Login here. DOM-based XSS is an advanced XSS attack. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. Please insert your password to refresh your session. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. Semgrep rule to identify above dom xss link. 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. Output Encoding and HTML Sanitization help address those gaps. We want to hear from you! Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Tag helpers will also encode input you use in tag parameters. 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. 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. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. DOM-based cross-site scripting happens when data from a user controlled, Most of the violations like this can also be detected by running a code linter or, If the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. Each variable used in the user interface should be passed through an output encoding function. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. 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 (#). Variables should only be placed in a CSS property value. This is where Output Encoding and HTML Sanitization are critical. 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. DOM-based XSS Examples. Reduce the DOM XSS attack surface of your application. A list of safe HTML attributes is provided in the Safe Sinks section. Safe HTML Attributes include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. If A is double JavaScript encoded then the following if check will return false. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. At a basic level XSS works by tricking your application into inserting a