Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. 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). Read the entire Acunetix Web Application Vulnerability Report. Each parser has distinct and separate semantics in the way they can possibly execute script code which make creating consistent rules for mitigating vulnerabilities in various contexts difficult. See what Acunetix Premium can do for you. DOM-based XSS: DOM-based XSS occurs when an . It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. 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. Reduce risk. For example, websites often reflect URL parameters in the HTML response from the server. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". In many cases the context isn't always straightforward to discern. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. You might find that the source gets assigned to other variables. Input validation. DOM-based XSS attacks seek to exploit the DOM in a simple two step process: Create a Source: Inject a malicious script into a property found to be suceptible to DOM-based XSS attacks. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. This is where Output Encoding and HTML Sanitization are critical. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). Stored XSS is considered the most damaging type of XSS attack. Safe list ranges are specified as Unicode code charts, not languages. Testing JavaScript execution sinks for DOM-based XSS is a little harder. //The following DOES WORK because the encoded value is a valid variable name or function reference. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). All other contexts are unsafe and you should not place variable data in them. This variable includes some characters which are used in XSS attacks, namely <, " and >. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. On the client side, the HTTP response does not change but the script executes in malicious manner. However the opposite is the case with HTML encoding. Never rely on validation alone. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. 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. DOM-based cross-site scripting is the de-facto name for XSS bugs that are the result of active browser-side content on a page, typically JavaScript, obtaining user input and then doing something unsafe with it, leading to the execution of injected code. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. In this section, we'll describe DOM-based cross-site scripting (DOM XSS), explain how to find DOM XSS vulnerabilities, and talk about how to exploit DOM XSS with different sources and sinks. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. Get help and advice from our experts on all things Burp. When a browser is rendering HTML and any other associated content like CSS or JavaScript, it identifies various rendering contexts for the different kinds of input and follows different rules for each context. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. 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. Doing so encourages designs in which the security rules are close to the data that they process, where you have the most context to correctly sanitize the value. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. 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. Level up your hacking and earn more bug bounties. \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');",