dom based cross site scripting prevention

For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. . For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. Get started with Burp Suite Enterprise Edition. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Level up your hacking and earn more bug bounties. A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. 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. All the Acunetix developers come with years of experience in the web security sphere. Always encode untrusted input before output, no matter what validation or sanitization has been performed. I will show you three examples of DOM-based XSS attacks in this article. If these methods are provided with untrusted input, then an XSS vulnerability could result. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. XSS is one of the most common and dangerous web vulnerabilities, and it is . Its the same with computer security. After the page's JavaScript applies this malicious URL to the back link's href, clicking on the back link will execute it: Another potential sink to look out for is jQuery's $() selector function, which can be used to inject malicious objects into the DOM. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. 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 is a Safe Sink and will automatically URL encode data in it. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Avoid treating untrusted data as code or markup within JavaScript code. This cheat sheet provides guidance to prevent XSS vulnerabilities. Encode all characters using the \xHH format. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Validate all data that flows into your application from the server or a third-party API. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Other CSS Contexts are unsafe and you should not place variable data in them. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. If this isn't possible, then ensure the data is JavaScript encoded. 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. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. The enterprise-enabled dynamic web vulnerability scanner. There are numerous methods which implicitly eval() data passed to it that must be avoided. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. Cross-site scripting ( XSS) vulnerabilities first became known through the CERT Advisory CA-2000-02 (Malicious HTML Tags Embedded in Client Web Requests), although these vulnerabilities had been exploited before. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. . Content Security Policy - An allowlist that prevents content being loaded. In JavaScript code, the main context is JavaScript but with the right tags and context closing characters, an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. We want to hear from you! placed in an HTML Attribute. Event handlers such as onload and onerror can be used in conjunction with these elements. 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. However, depending on the tag which innerText is applied, code can be executed. Sometimes you can't change the offending code. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. In those cases, create a Trusted Type object yourself. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. HTML encoding takes characters such as < and changes them into a safe form like < Before putting untrusted data into an HTML attribute ensure it's HTML encoded. Each variable in a web application needs to be protected. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. As with all other Cross-site Scripting (XSS) vulnerabilities, this type of attack also relies on insecure handling of user input on an HTML page. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Output Encoding. 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. A script within the later response contains a sink which then processes the data in an unsafe way. //The following does NOT work because the event handler is being set to a string. Cross-Site Scripting (XSS) is a misnomer. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. 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()). Do your applications use this vulnerable package? See how our software enables the world to secure the web. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). //The following does NOT work because of the encoded ";". There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Learn more about types of cross-site scripting attacks The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. Get the latest content on web security in your inbox each week. There will be situations where you use a URL in different contexts. This is a Safe Sink and will automatically CSS encode data in it. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. If you directly access an encoder via System.Text.Encodings.Web. Output encoding is not perfect. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. If you must, the following examples describe some approaches that do and do not work. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. 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. All of this code originates on the server, which means it is the application owner's responsibility to make it safe from XSS, regardless of the type of XSS flaw it is. If you're using JavaScript to change a CSS property, look into using style.property = x. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Reduce the DOM XSS attack surface of your application. A Computer Science portal for geeks. . DOM-based XSS: In this type of attack, the attacker injects malicious code into a web page that is executed on the client-side within the Document Object Model (DOM) of the web page. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. If A is double JavaScript encoded then the following if check will return false. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. Using the right combination of defensive techniques is necessary to prevent XSS. 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Cross-Site Scripting (XSS) is a misnomer. The name originated from early versions of the attack where stealing data cross-site was the primary focus. 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. There are two ways to do this. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. Get your questions answered in the User Forum. 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. For each potential source, such as location, you first need to find cases within the page's JavaScript code where the source is being referenced. 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. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. 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. 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. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. DOM based XSS vulnerabilities therefore have to be prevented on the client side. WAFs also dont address the root cause of an XSS vulnerability. The best manual tools to start web security testing. There are several methods and attributes which can be used to directly render HTML content within JavaScript. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Accelerate penetration testing - find more bugs, more quickly. For instance, jQuery's attr() function can change the attributes of DOM elements. Browsers change functionality and bypasses are being discovered regularly. For example. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. All other contexts are unsafe and you should not place variable data in them. 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. DOM-based XSS vulnerabilities usually arise when JavaScript takes data from an attacker-controllable source, such as the URL, and passes it to a sink that supports dynamic code execution, such as eval () or innerHTML. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. XSS sinks are places where variables are placed into your webpage. Despite being rare, they may cause serious problems and only a few scanners can detect them. By default encoders use a safe list limited to the Basic Latin Unicode range and encode all characters outside of that range as their character code equivalents. A list of safe HTML attributes is provided in the Safe Sinks section. In many cases the context isn't always straightforward to discern.

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