or . However, you may still find vulnerable code in the wild. This is where Output Encoding and HTML Sanitization are critical. 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. Also, XSS attacks always execute in the browser. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. 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. 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 payload can be manipulated to deface the target application using a prompt that states: Your session has expired. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. OWASP recommends these in all circumstances. Please insert your password to refresh your session. Any variable that does not go through this process is a potential weakness. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. Consider adopting the following controls in addition to the above. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. In other words, add a level of indirection between untrusted input and specified object properties. Please note, it is always dangerous design to put untrusted data directly into a command execution context. If you directly access an encoder via System.Text.Encodings.Web. 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()). . It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. 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. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. It is a simple yet effective way to harvest passwords using only the victims browser. One example of an attribute which is thought to be safe is innerText. 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. For example, Acunetix. 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. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. The most common one would be adding it to an href or src attribute of an tag. Use a trusted and verified library to escape HTML inputs. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. Enhance security monitoring to comply with confidence. Use only safe functions like document.innerText and document.textContent. CSS Contexts refer to variables placed into inline CSS. document.CreateTextNode () and append it in the appropriate DOM location. (It's free!). 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. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. \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');",