dom based cross site scripting prevention

Some pure DOM-based vulnerabilities are self-contained within a single page. Cross-Site Scripting (XSS) Attacks & How To Prevent Them Also, XSS attacks always execute in the browser. XSS sinks are places where variables are placed into your webpage. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Output encoding is the primary defense against cross-site scripting vulnerabilities. DOM Based Attacks. DOM-based cross-site scripting attack DOM-based XSS is also sometimes called "type-0 XSS." It occurs when the XSS vector executes as a result of a DOM modification on a website in a user's browser. Some examples of DOM-based XSS attacks include: 1. Customization of the safe list only affects encoders sourced via DI. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC 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. Read more about DOM-based cross-site scripting. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. There will be situations where you use a URL in different contexts. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. The reasoning behind this is to protect against unknown or future browser bugs (previous browser bugs have tripped up parsing based on the processing of non-English characters). 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. Fewer XSS bugs appear in applications built with modern web frameworks. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Any variable that does not go through this process is a potential weakness. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Reduce risk. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. The name originated from early versions of the attack where stealing data cross-site was the primary focus. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). What is DOM-based cross-site scripting? - PortSwigger . Therefore, the primary recommendation is to avoid including untrusted data in this context. 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. The HTML parser of the rendering context dictates how data is presented and laid out on the page and can be further broken down into the standard contexts of HTML, HTML attribute, URL, and CSS. In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template. An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. In these cases, HTML Sanitization should be used. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. Once you've found where the source is being read, you can use the JavaScript debugger to add a break point and follow how the source's value is used. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. It will not always prevent XSS. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. The other alternative is using N-levels of encoding. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. Encoding libraries often have a EncodeForJavaScript or similar to support this function. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". There are a couple of options for fixing a Trusted Type violation. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. DOM XSS stands for Document Object Model-based Cross-site Scripting. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. Download the latest version of Burp Suite. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. 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. DOM-based XSS is an advanced XSS attack. There are two distinct groups of cross-site scripting. Any application is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can develop from source to sink. Read more about DOM-based cross-site scripting. This means you will need to use alternative elements like img or iframe. Stored XSS is considered the most damaging type of XSS attack. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. Reduce the DOM XSS attack surface of your application. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. Validation becomes more complicated when accepting HTML in user input. XSS Prevention & Mitigation. Now only JavaScript encoding on server side. 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. The DOM-based cross-site scripting requires the user to open an infected page. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Normally executing JavaScript from a CSS context required either passing javascript:attackCode() to the CSS url() method or invoking the CSS expression() method passing JavaScript code to be directly executed. You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. DOM-Based Cross-Site Scripting (DOM XSS) | Learn AppSec - Invicti Trusted Types force you to process a value. 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); }. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. 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. WAFs are unreliable and new bypass techniques are being discovered regularly. Using untrusted user data on the left side of the expression allows an attacker to subvert internal and external attributes of the window object, whereas using user input on the right side of the expression doesn't allow direct manipulation. An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. The good news is that if user input is handled properly at the foundation level (e.g. If you must, the following examples describe some approaches that do and do not work. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. How To Prevent DOM-based Cross-site Scripting - emtmeta.com Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Websites may also store data on the server and reflect it elsewhere. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. Acunetix developers and tech agents regularly contribute to the blog. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. The line above could have possibly worked to render a link. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. Cross Site Scripting (XSS) | OWASP Foundation Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability, is the code in one of your policies - and you can lock that down even more by limiting policy creation. 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. In other words, add a level of indirection between untrusted input and specified object properties. The data is subsequently read from the DOM by the web application and outputted to the browser. Read the entire Acunetix Web Application Vulnerability Report. On the client side, the HTTP response does not change but the script executes in malicious manner. The third cross site scripting attack occurs entirely in the browser. Always pass untrusted input as a query string value. The DOM is a programming interface. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Browsers change functionality and bypasses are being discovered regularly. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). (It's free!). Don't use untrusted input as part of a URL path. For example, Acunetix. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Don't mutate DOM directly. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). Learn more about types of cross-site scripting attacks Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. React XSS Guide: Examples and Prevention - StackHawk 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. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. Variables should not be interpreted as code instead of text. 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. Get the latest content on web security in your inbox each week. In many cases, JavaScript encoding does not stop attacks within an execution context. 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. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. 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. There are two ways to do this. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. We will look at eval, href and dangerouslySetHTML vulnerabilities. That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. It is also impossible to protect against such client-side attacks using WAFs. The styling will not be rendered. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. For that, first create a policy. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. 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. 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. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. A rendering context is associated with the parsing of HTML tags and their attributes. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. In DOM-based cross-site scripting, the HTML source code and response of the attack . //The following does NOT work because of the encoded "(" and ")". //The following DOES WORK because the encoded value is a valid variable name or function reference. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. *Encoder.Default then the default, Basic Latin only safelist will be used. document.CreateTextNode () and append it in the appropriate DOM location. A Complete Guide To Cross Site Scripting - fas3c7.blogspot.com It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. Otherwise, again, your security efforts are void. We want to hear from you! What's the best way to prevent XSS attacks? | TechTarget Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Never rely on validation alone. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. This means, that no data will be available in server logs. DOM based XSS vulnerabilities therefore have to be prevented on the client side. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. What is XSS? Impact, Types, and Prevention - Bright Security We are looking for web developers to participate in user research, product testing, discussion groups and more. Examples of safe attributes includes: 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. Developers should use the following prevention steps to avoid introducing XSS into their application. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Document Object Model (DOM) Based XSS. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. It uses HTML attribute encoding rules whenever you use the @ directive. One example of an attribute which is thought to be safe is innerText. 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. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). If your code looked like the following, you would need to only double JavaScript encode input data. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Tag helpers will also encode input you use in tag parameters. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. Thankfully, many sinks where variables can be placed are safe. It's important to remember that some of these are also potential sources and sinks for DOM XSS. WSTG - v4.1 | OWASP Foundation For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS.

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