On the client side, the HTTP response does not change but the script executes in malicious manner. Scale dynamic scanning. 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. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. 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. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. XSS is one of the most common and dangerous web vulnerabilities, and it is . DOM Based Attacks. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. Avoid treating untrusted data as code or markup within JavaScript code. . // is an example of untrusted data that was properly JavaScript encoded but still executes. Don't use untrusted input as part of a URL path. . If you pollute a river, it'll flow downstream somewhere. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. placed in an HTML Attribute. Use only safe functions like document.innerText and document.textContent. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. This view outputs the contents of the untrustedInput variable. XSS Prevention & Mitigation. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. 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. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. I will show you three examples of DOM-based XSS attacks in this article. This brings up an interesting design point. The DOM is a programming interface. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. Output encoding is not perfect. Browsers change functionality and bypasses are being discovered regularly. 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 majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. This could lead to an attack being added to a webpage.. for example. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. The best manual tools to start web security testing. Encode all characters with the %HH encoding format. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. Some examples of DOM-based XSS attacks include: 1. There will be times where you need to do something outside the protection provided by your framework. For that, first create a policy. 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 is because these sinks treat the variable as text and will never execute it. These locations are known as dangerous contexts. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. 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. 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. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. OWASP recommends these in all circumstances. DOM-based XSS is a kind of XSS occurring entirely on the client-side. Validate all data that flows into your application from the server or a third-party API. The name originated from early versions of the attack where stealing data cross-site was the primary focus. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. - owasp-CheatSheetSeries . 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. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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. Thankfully, many sinks where variables can be placed are safe. 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. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Encode all characters using the \xHH format. 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). There are a variety of sinks that are relevant to DOM-based vulnerabilities. How common is DOM-based cross-site scripting? To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Always pass untrusted input as a query string value. 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. 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. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. There are some further things to consider: Security professionals often talk in terms of sources and sinks. document.CreateTextNode () and append it in the appropriate DOM location. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. 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. It is a simple yet effective way to harvest passwords using only the victims browser. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. 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. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. 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. In these cases, HTML Sanitization should be used. 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. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Common injection vectors include document.url, document.location, and document.referrer objects. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. 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. Also, XSS attacks always execute in the browser. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. A rendering context is associated with the parsing of HTML tags and their attributes. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. For details, see the Google Developers Site Policies. Tag helpers will also encode input you use in tag parameters. Before putting untrusted data into a URL query string ensure it's URL encoded. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. 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. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Otherwise, again, your security efforts are void. Misconceptions abound related to the proper encoding that is required. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. 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. 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 good news is that if user input is handled properly at the foundation level (e.g. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. Trusted Types force you to process a value. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. . If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. 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. 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. Reduce risk. We are looking for web developers to participate in user research, product testing, discussion groups and more. We want to hear from you! Want to track your progress and have a more personalized learning experience? Now only JavaScript encoding on server side. Output encoding is the primary defense against cross-site scripting vulnerabilities. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. 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. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. Validation becomes more complicated when accepting HTML in user input. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. 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.