dom based cross site scripting prevention

If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Trusted Types force you to process a value. Before putting untrusted data inside an HTML element ensure it's HTML encoded. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Don't use untrusted input as part of a URL path. The third cross site scripting attack occurs entirely in the browser. Encode all characters using the \xHH format. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. The primary difference is where the attack is injected into the application. It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. In many cases, JavaScript encoding does not stop attacks within an execution context. 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. Already got an account? 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. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Never rely on validation alone. 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. 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. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Avoid treating untrusted data as code or markup within JavaScript code. There are some further things to consider: Security professionals often talk in terms of sources and sinks. Otherwise, again, your security efforts are void. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. For details, see the Google Developers Site Policies. //The following does NOT work because the event handler is being set to a string. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. For example. Stored XSS is considered the most damaging type of XSS attack. Want to track your progress and have a more personalized learning experience? This is because these sinks treat the variable as text and will never execute it. 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. We will look at eval, href and dangerouslySetHTML vulnerabilities. This is why you would need to HTML encode too. Safe list ranges are specified as Unicode code charts, not languages. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. See how our software enables the world to secure the web. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Websites may also store data on the server and reflect it elsewhere. This variable includes some characters which are used in XSS attacks, namely <, " and >. See what Acunetix Premium can do for you. There will be situations where you use a URL in different contexts. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. The other alternative is using N-levels of encoding. I will show you three examples of DOM-based XSS attacks in this article. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. 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. The #redir route is executed by another file, redir.html. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. However, you may still find vulnerable code in the wild. 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. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. . Browsers change functionality and bypasses are being discovered regularly. Login here. 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. The good news is that if user input is handled properly at the foundation level (e.g. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Each variable in a web application needs to be protected. Thankfully, many sinks where variables can be placed are safe. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). eval You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. Common injection vectors include document.url, document.location, and document.referrer objects. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. It is also impossible to protect against such client-side attacks using WAFs. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Avoid populating the following methods with untrusted data. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. Get your questions answered in the User Forum. 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. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. The best manual tools to start web security testing. 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. Output encoding is the primary defense against cross-site scripting vulnerabilities. your framework), you should be able to mitigate all XSS vulnerabilities. OWASP recommends these in all circumstances. Get started with Burp Suite Enterprise Edition. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. 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. 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. //The following does NOT work because of the encoded ";". WAFs are unreliable and new bypass techniques are being discovered regularly. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. The world's #1 web penetration testing toolkit. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. If this isn't possible, then ensure the data is JavaScript encoded. 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 rendered output would now become. The HTML encoded value above is still executable. 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()). To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. XSS Prevention & Mitigation. 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). 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. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. In some . The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. The best way to fix DOM based cross-site scripting is to use the right output method (sink). HTML encoding takes characters such as < and changes them into a safe form like < Before putting untrusted data into an HTML attribute ensure it's HTML encoded. Automatic encoding and escaping functions are built into most frameworks. CSS is surprisingly powerful and has been used for many types of attacks. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. How to prevent DOM-based cross-site scripting? 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. This helps quickly identify a large chunk of violations. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. (It's free!). XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. 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. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). The following are some of the main sinks that can lead to DOM-XSS vulnerabilities: The following jQuery functions are also sinks that can lead to DOM-XSS vulnerabilities: In addition to the general measures described on the DOM-based vulnerabilities page, you should avoid allowing data from any untrusted source to be dynamically written to the HTML document. CSS Contexts refer to variables placed into inline CSS. All other contexts are unsafe and you should not place variable data in them. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . Please insert your password to refresh your session. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. 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. 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. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. DOM-based XSS is an advanced XSS attack. 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. 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. If that isn't enough to keep in mind, you have to remember that encodings are lost when you retrieve them using the value attribute of a DOM element. Record your progression from Apprentice to Expert. This cheat sheet provides guidance to prevent XSS vulnerabilities. Trusted Types heavily reduce the DOM XSS attack surface of your application. Please note, element.setAttribute is only safe for a limited number of attributes. Read about other types of cross-site scripting attacks. The DOM is a programming interface. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. 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. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. The DOM-based cross-site scripting requires the user to open an infected page. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. 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. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. At a basic level XSS works by tricking your application into inserting a