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. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. 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. 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. DOM based cross site scripting (Video solution) - YouTube 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. Cross Site Scripting Prevention Cheat Sheet - OWASP Understanding the XSS Threat: A Comprehensive Guide to DOM Based Cross It is always a bad idea to use a user-controlled input in dangerous sources such as eval. The web application dynamically generates a web page that contains this untrusted data. The line above could have possibly worked to render a link. 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). There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. How DOM Based XSS Attacks work - Bright Security All other contexts are unsafe and you should not place variable data in them. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. Stored XSS is considered the most damaging type of XSS attack. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. It is the process of converting untrusted . The enterprise-enabled dynamic web vulnerability scanner. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. This brings up an interesting design point. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. How To Prevent DOM-based Cross-site Scripting | Acunetix If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Cross-Site Scripting (XSS) is a misnomer. - owasp-CheatSheetSeries . 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. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. How to prevent cross-site scripting attacks | Infosec Resources 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 only safe location for placing variables in JavaScript is inside a quoted data value. 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. What is Cross-Site Scripting (XSS)? How to Prevent it? | Fortinet 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. Trusted Types require you to process the data before passing it to the above sink functions. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). In JavaScript code, the main context is JavaScript but with the right tags and context closing characters, an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods. What is WordPress Cross-site Scripting (XSS) and How to prevent it? 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. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. The third cross site scripting attack occurs entirely in the browser. You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Get started with Burp Suite Professional. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. Before putting untrusted data inside an HTML element ensure it's HTML encoded. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. 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. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). 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. 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. Validation can be a useful tool in limiting XSS attacks. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. There are a couple of options for fixing a Trusted Type violation. Tag helpers will also encode input you use in tag parameters. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. Get the latest content on web security in your inbox each week. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Cross Site Scripting (XSS) | OWASP Foundation There are 3 primary types of cross-site scripting: DOM-based XSS. . An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. placed in an HTML Attribute. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. Read more about DOM-based cross-site scripting. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? This is a Safe Sink and will automatically URL encode data in it. 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. Sometimes you can't change the offending code. Read the entire Acunetix Web Application Vulnerability Report. Doing so encourages designs in which the security rules are close to the data that they process, where you have the most context to correctly sanitize the value. Acunetix developers and tech agents regularly contribute to the blog. 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. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Catch critical bugs; ship more secure software, more quickly. In DOM-based cross-site scripting, the HTML source code and response of the attack . 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. 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. Cross Site Scripting PreventionProtect and Prevent XSS This is because these sinks treat the variable as text and will never execute it. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. 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. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. OWASP TOP 10: Cross-site scripting (XSS) ~2023 | Udemy A list of safe HTML attributes is provided in the Safe Sinks section. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. 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. 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. Otherwise, again, your security efforts are void. What is Cross-Site Scripting (XSS) and How to Prevent It? A script within the later response contains a sink which then processes the data in an unsafe way. DOM XSS stands for Document Object Model-based Cross-site Scripting. Accelerate penetration testing - find more bugs, more quickly. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. These locations are known as dangerous contexts. Read about other types of cross-site scripting attacks. 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 other alternative is using N-levels of encoding. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. 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. For DOM XSS, the attack is injected into the application during runtime in the client directly. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. When this happens, a script on the web page selects the URL variable and executes the code it contains. Variables should only be placed in a CSS property value. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. 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. How To Prevent DOM-based Cross-site Scripting - emtmeta.com Level up your hacking and earn more bug bounties. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. There are two distinct groups of cross-site scripting. The most common one would be adding it to an href or src attribute of an tag. Note that browsers behave differently with regards to URL-encoding, Chrome, Firefox, and Safari will URL-encode location.search and location.hash, while IE11 and Microsoft Edge (pre-Chromium) will not URL-encode these sources. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. There will be times where you need to do something outside the protection provided by your framework. 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. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". CSS Contexts refer to variables placed into inline CSS. Testing JavaScript execution sinks for DOM-based XSS is a little harder. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. DOM-based XSS: DOM-based XSS occurs when an . XSS sinks are places where variables are placed into your webpage. Thankfully, many sinks where variables can be placed are safe. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. A rendering context is associated with the parsing of HTML tags and their attributes. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. 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. 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. Already got an account? This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. This can be done via a function such as: 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. Its the same with computer security. Event handlers such as onload and onerror can be used in conjunction with these elements. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. 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. How to detect DOM-based cross-site scripting? In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. Cross-Site Scripting (XSS) Attacks & How To Prevent Them (It's free!). These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Trusted Types work by locking down the following risky sink functions. 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. 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. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Read more about DOM-based cross-site scripting. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. It is also impossible to protect against such client-side attacks using WAFs. You might find that the source gets assigned to other variables. A Complete Guide To Cross Site Scripting - fas3c7.blogspot.com 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. Fewer XSS bugs appear in applications built with modern web frameworks. In many cases, JavaScript encoding does not stop attacks within an execution context. Content Security Policy - An allowlist that prevents content being loaded. Record your progression from Apprentice to Expert. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. The best manual tools to start web security testing. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. For example, a numeric string containing only the characters 0-9 won't trigger an XSS attack. DOM-Based Cross-Site Scripting. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. If this isn't possible, then ensure the data is JavaScript encoded. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. The logic which parses URLs in both execution and rendering contexts looks to be the same. 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. //The following does NOT work because the event handler is being set to a string. 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. \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');",