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. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Get started with Burp Suite Professional. Validate all data that flows into your application from the server or a third-party API. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. This is common when you want users to be able to customize the look and feel of their webpages. It is a simple yet effective way to harvest passwords using only the victims browser. The problem is that if companyName had the value "Johnson & Johnson". Always encode untrusted input before output, no matter what validation or sanitization has been performed. Use a CSP as an additional layer of defense and have a look at the. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". However, if the pages returned from your web application utilize a content type of text/xhtml or the file type extension of *.xhtml then HTML encoding may not work to mitigate against XSS. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. Input validation. What would be displayed in the input text field would be "Johnson & Johnson". There are 3 primary types of cross-site scripting: DOM-based XSS. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. See Browser compatibility for up-to-date cross-browser support information.Key TermDOM-based cross-site scripting happens when data from a user controlled source (like user name, or redirect URL taken from the URL fragment) reaches a sink, which is a function like eval() or a property setter like .innerHTML, that can execute arbitrary JavaScript code. All other contexts are unsafe and you should not place variable data in them. It's important to remember that some of these are also potential sources and sinks for DOM XSS. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. If your code looked like the following, you would need to only double JavaScript encode input data. Any variable that does not go through this process is a potential weakness. Trusted Types heavily reduce the DOM XSS attack surface of your application. Event handlers such as onload and onerror can be used in conjunction with these elements. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. 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 certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. For DOM XSS, the attack is injected into the application during runtime in the client directly. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. Common injection vectors include document.url, document.location, and document.referrer objects. This is because these sinks treat the variable as text and will never execute it. Trusted Types work by locking down the following risky sink functions. The DOM-based cross-site scripting requires the user to open an infected page. Save time/money. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. Sometimes you can't change the offending code. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. You need to work through each available source in turn, and test each one individually. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Despite being rare, they may cause serious problems and only a few scanners can detect them. Each encoder, Html, JavaScript and Url, must be configured separately. For instance, jQuery's attr() function can change the attributes of DOM elements. Don't use untrusted input as part of a URL path. The world's #1 web penetration testing toolkit. Output Encoding. //The following does NOT work because of the encoded ";". It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. 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. How common is DOM-based cross-site scripting? If A is double JavaScript encoded then the following if check will return false. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. 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 payload can be manipulated to deface the target application using a prompt that states: Your session has expired. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. It is the process of converting untrusted . However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. Please refer to the list below for details. 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. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. If you're using JavaScript for writing to HTML, look at the .textContent attribute as it is a Safe Sink and will automatically HTML Entity Encode. Encode all characters with the %HH encoding format. The Unicode standard has a list of code charts you can use to find the chart containing your characters. Fewer XSS bugs appear in applications built with modern web frameworks. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. However, you may still find vulnerable code in the wild. If you must, the following examples describe some approaches that do and do not work. *Encoder.Default then the default, Basic Latin only safelist will be used. There are numerous methods which implicitly eval() data passed to it that must be avoided. In the case above, JavaScript encoding does not mitigate against DOM based XSS. Semgrep rule to identify above dom xss link. 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. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. //The following does NOT work because the event handler is being set to a string. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. This is a Safe Sink and will automatically CSS encode data in it. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. Want to track your progress and have a more personalized learning experience? Learn more about types of cross-site scripting attacks Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. 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. 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. So XSS has already been around for a while. Get the latest content on web security in your inbox each week. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS.