Read the entire Acunetix Web Application Vulnerability Report. 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()). The rendered output would now become. The logic which parses URLs in both execution and rendering contexts looks to be the same. One example of an attribute which is thought to be safe is innerText. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Customization of the safe list only affects encoders sourced via DI. 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. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. 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. Before putting untrusted data inside an HTML element ensure it's HTML encoded. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. CSS Contexts refer to variables placed into inline CSS. 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. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. There are some further things to consider: Security professionals often talk in terms of sources and sinks. It will not always prevent XSS. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. . In those cases, create a Trusted Type object yourself. 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. An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. 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. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. WAFs are unreliable and new bypass techniques are being discovered regularly. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. Event handlers such as onload and onerror can be used in conjunction with these elements. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. 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. HTML Context refers to inserting a variable between two basic HTML tags like a
or
. There are two distinct groups of cross-site scripting. It is the process of converting untrusted . We will look at eval, href and dangerouslySetHTML vulnerabilities. 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. 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. 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. How to prevent DOM-based cross-site scripting? If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. The best way to fix DOM based cross-site scripting is to use the right output method (sink). For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. This is because these sinks treat the variable as text and will never execute it. There may be times you want to insert a value into JavaScript to process in your view. The line above could have possibly worked to render a link. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. . Otherwise, again, your security efforts are void. 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 are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. 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. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. If your code looked like the following, you would need to only double JavaScript encode input data. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. What's the difference between Pro and Enterprise Edition? Stored XSS is considered the most damaging type of XSS attack. Learn more about types of cross-site scripting attacks 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. 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. For details, see the Google Developers Site Policies. 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. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. There are a variety of sinks that are relevant to DOM-based vulnerabilities. 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. It's important to remember that some of these are also potential sources and sinks for DOM XSS. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. Trusted Types force you to process a value. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. Output encoding is not perfect. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. The HTML encoded value above is still executable. 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. Read more about DOM-based cross-site scripting. 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. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. For example. Cross-Site Scripting (XSS) is a misnomer. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. For example.. An attacker could modify data that is rendered as $varUnsafe. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. 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. On the client side, the HTTP response does not change but the script executes in malicious manner. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Accelerate penetration testing - find more bugs, more quickly. 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. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). 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. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. All the Acunetix developers come with years of experience in the web security sphere. Some pure DOM-based vulnerabilities are self-contained within a single page. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. A list of output encoding libraries is included in the appendix. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. 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. If you're using JavaScript to change a CSS property, look into using style.property = x. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. For example, Acunetix. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Get the latest content on web security in your inbox each week. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. Other CSS Contexts are unsafe and you should not place variable data in them. 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. Do your applications use this vulnerable package? The Unicode standard has a list of code charts you can use to find the chart containing your characters. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. No single technique will solve XSS. The DOM is a programming interface. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). Automatic encoding and escaping functions are built into most frameworks. *Encoder.Default then the default, Basic Latin only safelist will be used. 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. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. This is why you would need to HTML encode too. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. Summary. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. OWASP recommends DOMPurify for HTML Sanitization. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. 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. All of this code originates on the server, which means it is the application owner's responsibility to make it safe from XSS, regardless of the type of XSS flaw it is. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. 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. Download the latest version of Burp Suite. 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. This cheat sheet provides guidance to prevent XSS vulnerabilities. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. 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. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. 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). In this section, we'll describe DOM-based cross-site scripting (DOM XSS), explain how to find DOM XSS vulnerabilities, and talk about how to exploit DOM XSS with different sources and sinks. Variables should not be interpreted as code instead of text. 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=. Validation becomes more complicated when accepting HTML in user input. Practise exploiting vulnerabilities on realistic targets. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. 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. Don't mutate DOM directly. The styling will not be rendered. HTML tag elements are well defined and do not support alternate representations of the same tag. The primary difference is where the attack is injected into the application. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . HTML Attribute Contexts refer to placing a variable in an HTML attribute value. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. 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 . Avoid treating untrusted data as code or markup within JavaScript code. Input validation. your framework), you should be able to mitigate all XSS vulnerabilities. There are a couple of options for fixing a Trusted Type violation. How common is DOM-based cross-site scripting? //The following does NOT work because of the encoded ";". 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 (#). Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. - owasp-CheatSheetSeries . To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. We are looking for web developers to participate in user research, product testing, discussion groups and more. So XSS has already been around for a while. 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. 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. . Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. XSS Prevention & Mitigation. This means you will need to use alternative elements like img or iframe. For a comprehensive list, check out the DOMPurify allowlist. //The following does NOT work because of the encoded "(" and ")". Use URL Encoding for these scenarios. 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. Learn the details here including XSS prevention methods. There will be situations where you use a URL in different contexts. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. 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. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. Please note, it is always dangerous design to put untrusted data directly into a command execution context. 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. (It's free!). Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. 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. Output Encoding and HTML Sanitization help address those gaps. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Make sure any attributes are fully quoted, same as JS and CSS. 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. 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.