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The problem is that if companyName had the value "Johnson & Johnson". Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. The styling will not be rendered. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. More info about Internet Explorer and Microsoft Edge. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Always pass untrusted input as a query string value. 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. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. For that, first create a policy. Get your questions answered in the User Forum. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. For example, websites often reflect URL parameters in the HTML response from the server. Free, lightweight web application security scanning for CI/CD. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. DOM based XSS vulnerabilities therefore have to be prevented on the client side. Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. For a detailed explanation of the taint flow between sources and sinks, please refer to the DOM-based vulnerabilities page. For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. The enterprise-enabled dynamic web vulnerability scanner. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). For example. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. This can be done via a function such as: Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. In the case above, JavaScript encoding does not mitigate against DOM based XSS. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. The #redir route is executed by another file, redir.html. 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 logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. The logic which parses URLs in both execution and rendering contexts looks to be the same. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. Output Encoding and HTML Sanitization help address those gaps. Encode all characters using the \xHH format. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. 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. 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. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Catch critical bugs; ship more secure software, more quickly. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. CSS is surprisingly powerful and has been used for many types of attacks. 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. A list of output encoding libraries is included in the appendix. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. 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 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). DOM-based cross-site scripting is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. The DOM is a programming interface. Trusted Types require you to process the data before passing it to the above sink functions. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. //The following does NOT work because the event handler is being set to a string. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. We will look at eval, href and dangerouslySetHTML vulnerabilities. Learn the details here including XSS prevention methods. The primary difference is where the attack is injected into the application. \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');",