Avoid populating the following methods with untrusted data. This is common when you want users to be able to customize the look and feel of their webpages. It is the process of converting untrusted . Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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); }. //The following DOES WORK because the encoded value is a valid variable name or function reference. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. . This fact makes it more difficult to maintain web application security. 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()). 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. 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. These types of attacks typically occur as a result . Cross-Site Scripting (XSS) is a misnomer. Its critical to use quotation marks like " or ' to surround your variables. Validation can be a useful tool in limiting XSS attacks. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. 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. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Output encoding here will prevent XSS, but it will break the intended functionality of the application. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. 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 following does NOT work because of the encoded "(" and ")". Customization of the safe list only affects encoders sourced via DI. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. Login here. 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. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. One example of an attribute which is thought to be safe is innerText. Encode all characters with the %HH encoding format. No single technique will solve XSS. For example, you might need to close some existing elements before using your JavaScript payload. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. 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. 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. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. XSS sinks are places where variables are placed into your webpage. This can be done via a function such as: Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. Please refer to the list below for details. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. 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. For example: Modern web applications are typically built using a number of third-party libraries and frameworks, which often provide additional functions and capabilities for developers. If this isn't possible, then ensure the data is JavaScript encoded. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. Don't use untrusted input as part of a URL path. 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. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. This helps quickly identify a large chunk of violations. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. CSS Contexts refer to variables placed into inline CSS. *Encoder.Default then the default, Basic Latin only safelist will be used. The data is subsequently read from the DOM by the web application and outputted to the browser. HTML tag elements are well defined and do not support alternate representations of the same tag. More info about Internet Explorer and Microsoft Edge. 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. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? 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 most common one would be adding it to an href or src attribute of an tag. Safe list ranges are specified as Unicode code charts, not languages. The name originated from early versions of the attack where stealing data cross-site was the primary focus. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Thankfully, many sinks where variables can be placed are safe. Get your questions answered in the User Forum. Stored XSS is considered the most damaging type of XSS attack. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Script manipulation: <script src> and setting text content of <script> elements. There are some further things to consider: Security professionals often talk in terms of sources and sinks. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. 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. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. So XSS has already been around for a while. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. WAFs are unreliable and new bypass techniques are being discovered regularly. 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 example if you want to use user input to write in a div tag element don't use innerHtml, instead use innerText or textContent. Trusted Types work by locking down the following risky sink functions. 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. Consider adopting the following controls in addition to the above. The only safe location for placing variables in JavaScript is inside a quoted data value. Enhance security monitoring to comply with confidence. However, depending on the tag which innerText is applied, code can be executed. (It's free!). 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. 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. The web application dynamically generates a web page that contains this untrusted data. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. Encoding libraries often have a EncodeForJavaScript or similar to support this function. Get started with Burp Suite Professional. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. Developers should use the following prevention steps to avoid introducing XSS into their application. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. Already got an account? Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. 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). Testing JavaScript execution sinks for DOM-based XSS is a little harder. If you're using JavaScript to change a CSS property, look into using style.property = x. You should apply HTML attribute encoding to variables being placed in most HTML attributes. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. 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. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Browsers change functionality and bypasses are being discovered regularly. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. 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. By default encoders use a safe list limited to the Basic Latin Unicode range and encode all characters outside of that range as their character code equivalents. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. 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. The problem is that if companyName had the value "Johnson & Johnson". 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. 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. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. 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. 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 cheatsheet is a list of techniques to prevent or limit the impact of XSS. Misconceptions abound related to the proper encoding that is required. Input validation. In a DOM-based attacks, the HTTP response on the server side does not change. - owasp-CheatSheetSeries . URL Contexts refer to variables placed into a URL. Common injection vectors include document.url, document.location, and document.referrer objects. 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. Make sure any attributes are fully quoted, same as JS and CSS. Each variable in a web application needs to be protected. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Use a trusted and verified library to escape HTML inputs. 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. Want to track your progress and have a more personalized learning experience? It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). If these methods are provided with untrusted input, then an XSS vulnerability could result. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). There are 3 primary types of cross-site scripting: DOM-based XSS. Examples of safe attributes includes: 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. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. For each location where your string appears within the DOM, you need to identify the context. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Based on this context, you need to refine your input to see how it is processed. Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Please note, element.setAttribute is only safe for a limited number of attributes. Save time/money. The Unicode standard has a list of code charts you can use to find the chart containing your characters. Encode all characters using the \xHH format. See how our software enables the world to secure the web. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. If you directly access an encoder via System.Text.Encodings.Web. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. 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. 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. Perhaps the non-conforming functionality is not needed anymore or can be rewritten in a modern way without using the error-prone functions?Don'tel.innerHTML = '<img src=xyz.jpg>'; Doel.textContent = '';const img = document.createElement('img');img.src = 'xyz.jpg';el.appendChild(img); Some libraries already generate Trusted Types that you can pass to the sink functions. 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 means, that no data will be available in server logs. Catch critical bugs; ship more secure software, more quickly. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. Never rely on validation alone. Each variable used in the user interface should be passed through an output encoding function. All the Acunetix developers come with years of experience in the web security sphere. 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). In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Reduce the DOM XSS attack surface of your application. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. 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. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. Validate all data that flows into your application from the server or a third-party API. 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). Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . 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. In those cases, create a Trusted Type object yourself. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. It is also impossible to protect against such client-side attacks using WAFs.