Get the latest content on web security in your inbox each week. Practise exploiting vulnerabilities on realistic targets. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. WAFs also dont address the root cause of an XSS vulnerability. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. For example.. An attacker could modify data that is rendered as $varUnsafe. There are a variety of sinks that are relevant to DOM-based vulnerabilities. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Working example (no HTML encoding): Normally encoded example (Does Not Work DNW): HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW): If HTML encoding followed the same semantics as JavaScript encoding. There will be situations where you use a URL in different contexts. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. 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. HTML tag elements are well defined and do not support alternate representations of the same tag. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. Developers should use the following prevention steps to avoid introducing XSS into their application. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. What is XSS? Impact, Types, and Prevention - Bright Security A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. 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. For example, this is the case if you're loading a third-party library from a CDN. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Common injection vectors include document.url, document.location, and document.referrer objects. 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. DOM-based XSS is an advanced XSS attack. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. In these cases, HTML Sanitization should be used. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. 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. Websites may also store data on the server and reflect it elsewhere. If you must, the following examples describe some approaches that do and do not work. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. \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');",