For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC 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. 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. Despite being rare, they may cause serious problems and only a few scanners can detect them. 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. Each variable in a web application needs to be protected. Parsing HTML input is difficult, if not impossible. This document only discusses JavaScript bugs which lead to XSS. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. 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. If these methods are provided with untrusted input, then an XSS vulnerability could result. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. your framework), you should be able to mitigate all XSS vulnerabilities. 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. Each variable used in the user interface should be passed through an output encoding function. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. The line above could have possibly worked to render a link. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. Please note, it is always dangerous design to put untrusted data directly into a command execution context. For example, you might need to close some existing elements before using your JavaScript payload. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. The web application dynamically generates a web page that contains this untrusted data. The best manual tools to start web security testing. . There are several methods and attributes which can be used to directly render HTML content within JavaScript. CSS is surprisingly powerful and has been used for many types of attacks. 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. How to prevent DOM-based cross-site scripting? 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. HTML Context refers to inserting a variable between two basic HTML tags like a
dom based cross site scripting prevention
or . All the Acunetix developers come with years of experience in the web security sphere. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. 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. We will look at eval, href and dangerouslySetHTML vulnerabilities. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. This can be done via a function such as: The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. 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. Semgrep rule to identify above dom xss link. A script within the later response contains a sink which then processes the data in an unsafe way. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. 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. You should apply HTML attribute encoding to variables being placed in most HTML attributes. 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. 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. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. This fact makes it more difficult to maintain web application security. 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. If you must, the following examples describe some approaches that do and do not work. . Any variable that does not go through this process is a potential weakness. The logic which parses URLs in both execution and rendering contexts looks to be the same. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. Customization of the safe list only affects encoders sourced via DI. 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. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. A Computer Science portal for geeks. Encode all characters with the %HH encoding format. 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. DOM-based XSS Examples. Read more about DOM-based cross-site scripting. . There are two distinct groups of cross-site scripting. eval It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. URL Contexts refer to variables placed into a URL. No single technique will solve XSS. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. In these cases, HTML Sanitization should be used. Use URL Encoding for these scenarios. Fewer XSS bugs appear in applications built with modern web frameworks. This view outputs the contents of the untrustedInput variable. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. In other words, add a level of indirection between untrusted input and specified object properties. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. 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. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. This cheat sheet provides guidance to prevent XSS vulnerabilities. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. . The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Using the right combination of defensive techniques is necessary to prevent XSS. 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. 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. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. 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. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Don't mutate DOM directly. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. When this happens, a script on the web page selects the URL variable and executes the code it contains. OWASP recommends DOMPurify for HTML Sanitization. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Output encoding is the primary defense against cross-site scripting vulnerabilities. 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. If you're using JavaScript to change a CSS property, look into using style.property = x. 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. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. This brings up an interesting design point. 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. The DOM is a programming interface. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. 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. WAFs are unreliable and new bypass techniques are being discovered regularly. (It's free!). HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. The problem is that if companyName had the value "Johnson & Johnson". In some . This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Automatic encoding and escaping functions are built into most frameworks. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. 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 ">". There may be times you want to insert a value into JavaScript to process in your view. Validation becomes more complicated when accepting HTML in user input. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Document Object Model (DOM) Based XSS. How common is DOM-based cross-site scripting? It is also impossible to protect against such client-side attacks using WAFs. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. We are looking for web developers to participate in user research, product testing, discussion groups and more. 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. This is a Safe Sink and will automatically CSS encode data in it. Developers should use the following prevention steps to avoid introducing XSS into their application. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. DOM Based Attacks. Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Your application can be vulnerable to both reflected/stored XSS and DOM XSS. If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. 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. 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. For example, Acunetix. Some examples of DOM-based XSS attacks include: 1. Always pass untrusted input as a query string value. Misconceptions abound related to the proper encoding that is required. Safe list ranges are specified as Unicode code charts, not languages. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. If you pollute a river, it'll flow downstream somewhere. 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. In many cases, JavaScript encoding does not stop attacks within an execution context. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. Trusted Types require you to process the data before passing it to the above sink functions. Now only JavaScript encoding on server side. This variable includes some characters which are used in XSS attacks, namely <, " and >. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . \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');",