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. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. (It's free!). 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. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. 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. Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. What would be displayed in the input text field would be "Johnson & Johnson". The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The Unicode standard has a list of code charts you can use to find the chart containing your characters. 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. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. DOM-based XSS is an advanced XSS attack. Safe list ranges are specified as Unicode code charts, not languages. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). Don't mutate DOM directly. Accelerate penetration testing - find more bugs, more quickly. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. Misconceptions abound related to the proper encoding that is required. XSS Prevention & Mitigation. DOM-Based Cross-Site Scripting. All the Acunetix developers come with years of experience in the web security sphere. Before putting untrusted data into a URL query string ensure it's URL encoded. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. eval Each variable used in the user interface should be passed through an output encoding function. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Please insert your password to refresh your session. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. If this is the case, you'll need to use the search function again to track these variables and see if they're passed to a sink. 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. It will not always prevent XSS. This means, that no data will be available in server logs. If you pollute a river, it'll flow downstream somewhere. Use only safe functions like document.innerText and document.textContent. Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Despite being rare, they may cause serious problems and only a few scanners can detect them. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. Encoding libraries often have a EncodeForJavaScript or similar to support this function. 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. The enterprise-enabled dynamic web vulnerability scanner. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. //The following does NOT work because of the encoded ";". Never rely on validation alone. \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');",