There are two ways to do this. //The following DOES WORK because the encoded value is a valid variable name or function reference. XSS Prevention & Mitigation. Finally, to fix the problem in our initial code, instead of trying to encode the output correctly which is a hassle and can easily go wrong we would simply use element.textContent to write it in a content like this: It does the same thing but this time it is not vulnerable to DOM based cross-site scripting vulnerabilities. RULE #1 - HTML Escape then JavaScript Escape Before Inserting Untrusted Data into HTML Subcontext within the Execution Context, RULE #2 - JavaScript Escape Before Inserting Untrusted Data into HTML Attribute Subcontext within the Execution Context, RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context, RULE #4 - JavaScript Escape Before Inserting Untrusted Data into the CSS Attribute Subcontext within the Execution Context, RULE #5 - URL Escape then JavaScript Escape Before Inserting Untrusted Data into URL Attribute Subcontext within the Execution Context, RULE #6 - Populate the DOM using safe JavaScript functions or properties, RULE #7 - Fixing DOM Cross-site Scripting Vulnerabilities, Guidelines for Developing Secure Applications Utilizing JavaScript, GUIDELINE #1 - Untrusted data should only be treated as displayable text, GUIDELINE #2 - Always JavaScript encode and delimit untrusted data as quoted strings when entering the application when building templated JavaScript, GUIDELINE #3 - Use document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar to build dynamic interfaces, GUIDELINE #4 - Avoid sending untrusted data into HTML rendering methods, GUIDELINE #5 - Avoid the numerous methods which implicitly eval() data passed to it, Utilizing an Enclosure (as suggested by Gaz), GUIDELINE #6 - Use untrusted data on only the right side of an expression, GUIDELINE #7 - When URL encoding in DOM be aware of character set issues, GUIDELINE #8 - Limit access to object properties when using object[x] accessors, GUIDELINE #9 - Run your JavaScript in a ECMAScript 5 canopy or sandbox, GUIDELINE #10 - Don't eval() JSON to convert it to native JavaScript objects, Common Problems Associated with Mitigating DOM Based XSS, Insecure Direct Object Reference Prevention, Creative Commons Attribution 3.0 Unported License. Save time/money. For example, using the default configuration you might use a Razor HtmlHelper like so; When you view the source of the web page you will see it has been rendered as follows, with the Chinese text encoded; To widen the characters treated as safe by the encoder you would insert the following line into the ConfigureServices() method in startup.cs; This example widens the safe list to include the Unicode Range CjkUnifiedIdeographs. Variables should only be placed in a CSS property value. This document only discusses JavaScript bugs which lead to XSS. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. DOM based XSS vulnerabilities therefore have to be prevented on the client side. The third cross site scripting attack occurs entirely in the browser. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. For example, Acunetix. The world's #1 web penetration testing toolkit. Get started with Burp Suite Enterprise Edition. The enterprise-enabled dynamic web vulnerability scanner. The DOM is a programming interface. One of the simplest ways of doing this is to deliver your exploit via an iframe: In this example, the src attribute points to the vulnerable page with an empty hash value. This is a Safe Sink and will automatically CSS encode data in it. The example that follows illustrates using closures to avoid double JavaScript encoding. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. 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 (#). JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. Record your progression from Apprentice to Expert. 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. If you must, the following examples describe some approaches that do and do not work. Trusted Types force you to process a value. //The following does NOT work because the event handler is being set to a string. Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Acunetix developers and tech agents regularly contribute to the blog. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. If you need to render different content, use innerText instead of innerHTML. However, depending on the tag which innerText is applied, code can be executed. In these cases, HTML Sanitization should be used. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. If you pollute a river, it'll flow downstream somewhere. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. Trusted Types heavily reduce the DOM XSS attack surface of your application. Websites may also store data on the server and reflect it elsewhere. In those cases, create a Trusted Type object yourself. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. Safe list ranges are specified as Unicode code charts, not languages. Perhaps the non-conforming functionality is not needed anymore or can be rewritten in a modern way without using the error-prone functions?Don'tel.innerHTML = '<img src=xyz.jpg>'; Doel.textContent = '';const img = document.createElement('img');img.src = 'xyz.jpg';el.appendChild(img); Some libraries already generate Trusted Types that you can pass to the sink functions. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. If you're using JavaScript to change a CSS property, look into using style.property = x. 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. This variable includes some characters which are used in XSS attacks, namely <, " and >. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. Sometimes you can't change the offending code. Customization of the safe list only affects encoders sourced via DI. View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). Other CSS Contexts are unsafe and you should not place variable data in them. An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. Enhance security monitoring to comply with confidence. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. 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. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. Reduce risk. The best way to fix DOM based cross-site scripting is to use the right output method (sink). From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. Encode all characters with the %HH encoding format. 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. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. This fact makes it more difficult to maintain web application security. A rendering context is associated with the parsing of HTML tags and their attributes. Accelerate penetration testing - find more bugs, more quickly. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . URL Contexts refer to variables placed into a URL. Ideally, the correct way to apply encoding and avoid the problem stated above is to server-side encode for the output context where data is introduced into the application. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. Use a trusted and verified library to escape HTML inputs. 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. Output encoding here will prevent XSS, but it will break the intended functionality of the application. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. We will look at eval, href and dangerouslySetHTML vulnerabilities. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. Read more about DOM-based cross-site scripting. Its the same with computer security. HTML Context refers to inserting a variable between two basic HTML tags like a