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. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. 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). It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. 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. 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. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. You need to work through each available source in turn, and test each one individually. A list of safe HTML attributes is provided in the Safe Sinks section. 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); }. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. 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. Encode all characters using the \xHH format. The name originated from early versions of the attack where stealing data cross-site was the primary focus. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. Safe list ranges are specified as Unicode code charts, not languages. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. When this happens, a script on the web page selects the URL variable and executes the code it contains. 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. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. 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. Use URL Encoding for these scenarios. 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. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. Its the same with computer security. Each encoder, Html, JavaScript and Url, must be configured separately. 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. Some examples of DOM-based XSS attacks include: 1. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based 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. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? 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. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). DOM-based XSS is an advanced XSS attack. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. Get started with Burp Suite Professional. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. For instance, jQuery's attr() function can change the attributes of DOM elements. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC A rendering context is associated with the parsing of HTML tags and their attributes. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. The best way to fix DOM based cross-site scripting is to use the right output method (sink). 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. Catch critical bugs; ship more secure software, more quickly. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Any variable that does not go through this process is a potential weakness. This means, that no data will be available in server logs. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Login here. \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');",