dom based cross site scripting prevention

Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. The name originated from early versions of the attack where stealing data cross-site was the primary focus. This behavior was often implemented using a vulnerable hashchange event handler, similar to the following: As the hash is user controllable, an attacker could use this to inject an XSS vector into the $() selector sink. This means, that no data will be available in server logs. Automatic encoding and escaping functions are built into most frameworks. 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, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. Testing JavaScript execution sinks for DOM-based XSS is a little harder. Trusted Types work by locking down the following risky sink functions. This variable includes some characters which are used in XSS attacks, namely <, " and >. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. Please insert your password to refresh your session. 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. These locations are known as dangerous contexts. Therefore, the primary recommendation is to avoid including untrusted data in this context. 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. 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. Document Object Model (DOM) Based XSS. Do your applications use this vulnerable package? OWASP recommends DOMPurify for HTML Sanitization. Trusted Types require you to process the data before passing it to the above sink functions. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. . This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. 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. Acunetix developers and tech agents regularly contribute to the blog. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC 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. Read the entire Acunetix Web Application Vulnerability Report. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. There may be times you want to insert a value into JavaScript to process in your view. Its critical to use quotation marks like " or ' to surround your variables. The web application dynamically generates a web page that contains this untrusted data. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). In DOM-based cross-site scripting, the HTML source code and response of the attack . Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. DOM-based XSS attacks seek to exploit the DOM in a simple two step process: Create a Source: Inject a malicious script into a property found to be suceptible to DOM-based XSS attacks. Get help and advice from our experts on all things Burp. Based on this context, you need to refine your input to see how it is processed. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. In these cases, HTML Sanitization should be used. The primary difference is where the attack is injected into the application. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Cookie Attributes - These change how JavaScript and browsers can interact with cookies. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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. 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. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. Learn more about types of cross-site scripting attacks This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against 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. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Other CSS Contexts are unsafe and you should not place variable data in them. Don't use untrusted input as part of a URL path. All the Acunetix developers come with years of experience in the web security sphere. Each encoder, Html, JavaScript and Url, must be configured separately. For example, you might need to close some existing elements before using your JavaScript payload. DOM-based XSS Examples. 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. So XSS has already been around for a while. 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. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! 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. 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. 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%>. // is an example of untrusted data that was properly JavaScript encoded but still executes. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. 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. If you must, the following examples describe some approaches that do and do not work. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Validation can be a useful tool in limiting XSS attacks. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. *Encoder.Default then the default, Basic Latin only safelist will be used. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. We will look at eval, href and dangerouslySetHTML vulnerabilities. For DOM XSS, the attack is injected into the application during runtime in the client directly. 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). This difference makes JavaScript encoding a less viable weapon in our fight against XSS. The encoder safe lists can be customized to include Unicode ranges appropriate to the app during startup, in Program.cs: For example, using the default configuration using a Razor HtmlHelper similar to the following: The preceding markup is rendered with Chinese text encoded: To widen the characters treated as safe by the encoder, insert the following line into Program.cs. Want to track your progress and have a more personalized learning experience? For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. This is a Safe Sink and will automatically CSS encode data in it. CSS is surprisingly powerful and has been used for many types of attacks. This is where Output Encoding and HTML Sanitization are critical. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. 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. Information on ordering, pricing, and more. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. There will be times where you need to do something outside the protection provided by your framework. How to detect DOM-based cross-site scripting? Output encoding here will prevent XSS, but it will break the intended functionality of the application. URL Contexts refer to variables placed into a URL. This brings up an interesting design point. . 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. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. A list of output encoding libraries is included in the appendix. . 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. In many cases the context isn't always straightforward to discern. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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. DOM based XSS vulnerabilities therefore have to be prevented on the client side. 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. The other alternative is using N-levels of encoding. Parsing HTML input is difficult, if not impossible. It uses HTML attribute encoding rules whenever you use the @ directive. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. There are two ways to do this. The attack functions by manipulating the internal model of the webpage within the browser known as the DOM and are referred to as DOM based attacks . This helps quickly identify a large chunk of violations. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Record your progression from Apprentice to Expert. eval A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Websites may also store data on the server and reflect it elsewhere. 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. If you need to render different content, use innerText instead of innerHTML. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". 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 is also impossible to protect against such client-side attacks using WAFs. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. 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). XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. When this happens, a script on the web page selects the URL variable and executes the code it contains. A list of safe HTML attributes is provided in the Safe Sinks section. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". 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. Already got an account? What's the difference between Pro and Enterprise Edition? WAFs also dont address the root cause of an XSS vulnerability. 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. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. Despite being rare, they may cause serious problems and only a few scanners can detect them. 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. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. See how our software enables the world to secure the web. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. 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=. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. 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. 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. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. The third cross site scripting attack occurs entirely in the browser. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. The DOM is a programming interface. For each location where your string appears within the DOM, you need to identify the context. Save time/money. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. XSS sinks are places where variables are placed into your webpage. DOM-based XSS is an advanced XSS attack. One example of an attribute which is thought to be safe is innerText. HTML Context refers to inserting a variable between two basic HTML tags like a