The complication is compounded by the differing meanings and treatment of encoded values within each subcontext (HTML, HTML attribute, URL, and CSS) within the execution context. DOM Based XSS | OWASP Foundation Practise exploiting vulnerabilities on realistic targets. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). The other alternative is using N-levels of encoding. This document only discusses JavaScript bugs which lead to XSS. Policies are factories for Trusted Types that enforce certain security rules on their input: This code creates a policy called myEscapePolicy that can produce TrustedHTML objects via its createHTML() function. DOM-Based Cross-Site Scripting. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. Automatic encoding and escaping functions are built into most frameworks. DOM-Based Cross-Site Scripting (DOM XSS) | Learn AppSec - Invicti All the Acunetix developers come with years of experience in the web security sphere. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. Using the right combination of defensive techniques is necessary to prevent XSS. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. Cross Site Scripting Prevention Cheat Sheet - github.com DOM-based XSS simply means a cross-site scripting vulnerability that occurs in the DOM ( Document Object Model) of your site rather than in HTML. The most common one would be adding it to an href or src attribute of an tag. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. What is DOM-based cross-site scripting? - PortSwigger A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript. 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. The DOM is a programming interface. Safe HTML Attributes include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. CWE - CWE-79: Improper Neutralization of Input During Web Page As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. Types of XSS (Cross-site Scripting) - Acunetix Learn more about types of cross-site scripting attacks 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. On the client side, the HTTP response does not change but the script executes in malicious manner. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. your framework), you should be able to mitigate all XSS vulnerabilities. Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). In the case above, JavaScript encoding does not mitigate against DOM based XSS. 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. 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. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. 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. Testing JavaScript execution sinks for DOM-based XSS is a little harder. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Want to track your progress and have a more personalized learning experience? Some pure DOM-based vulnerabilities are self-contained within a single page. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). At a basic level XSS works by tricking your application into inserting a