Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. 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. 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. 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()). Make sure any attributes are fully quoted, same as JS and CSS. 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. 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. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Note that the browser's "View source" option won't work for DOM XSS testing because it doesn't take account of changes that have been performed in the HTML by JavaScript. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. Get help and advice from our experts on all things Burp. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. The rendered output would now become. This fact makes it more difficult to maintain web application security. For instance, jQuery's attr() function can change the attributes of DOM elements. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. Websites may also store data on the server and reflect it elsewhere. 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. Trusted Types heavily reduce the DOM XSS attack surface of your application. 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. A rendering context is associated with the parsing of HTML tags and their attributes. It uses HTML attribute encoding rules whenever you use the @ directive. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: createElement () and assign property values with appropriate methods or properties such as node.textContent= or node.InnerText=. Other CSS Contexts are unsafe and you should not place variable data in them. 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 DOM-based cross-site scripting requires the user to open an infected page. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. 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. Learn the details here including XSS prevention methods. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). If A is double JavaScript encoded then the following if check will return false. 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. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. Cross-Site Scripting (XSS) is a misnomer. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. Document Object Model (DOM) Based XSS. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. 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 ">". 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. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. The other alternative is using N-levels of encoding. 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. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. 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 (#). You might find that the source gets assigned to other variables. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. CSS Contexts refer to variables placed into inline CSS. Validate all data that flows into your application from the server or a third-party API. Avoid populating the following methods with untrusted data. Scale dynamic scanning. Validation becomes more complicated when accepting HTML in user input. 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. Read more about DOM-based cross-site scripting. For details, see the Google Developers Site Policies. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. The best manual tools to start web security testing. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Parsing HTML input is difficult, if not impossible. In other words, add a level of indirection between untrusted input and specified object properties. This variable includes some characters which are used in XSS attacks, namely <, " and >. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. For DOM XSS, the attack is injected into the application during runtime in the client directly. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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. XSS sinks are places where variables are placed into your webpage. However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. Web Application Firewalls - These look for known attack strings and block them. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. Even newer versions of jQuery can still be vulnerable via the $() selector sink, provided you have full control over its input from a source that doesn't require a # prefix. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. These types of attacks typically occur as a result . Some examples of DOM-based XSS attacks include: 1. 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. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. . 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. Stored XSS is considered the most damaging type of XSS attack. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Validation can be a useful tool in limiting XSS attacks. If you sanitize content and then modify it afterwards, you can easily void your security efforts. DOM-Based Cross-Site Scripting. To detect the possibility of a DOM XSS, you must simulate the attack from the client-side in the users browser using a web application scanner like Acunetix (with DOM-based XSS scanner functionality). The reason why you only need to double JavaScript encode is that the customFunction function did not itself pass the input to another method which implicitly or explicitly called eval If firstName was passed to another JavaScript method which implicitly or explicitly called eval() then <%=doubleJavaScriptEncodedData%> above would need to be changed to <%=tripleJavaScriptEncodedData%>. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. This brings up an interesting design point. Any variable that does not go through this process is a potential weakness. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. What's the difference between Pro and Enterprise Edition? A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. jQuery used to be extremely popular, and a classic DOM XSS vulnerability was caused by websites using this selector in conjunction with the location.hash source for animations or auto-scrolling to a particular element on the page. DOM-based XSS Examples. So XSS has already been around for a while. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. In DOM-based cross-site scripting, the HTML source code and response of the attack . If you need to render different content, use innerText instead of innerHTML. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. In the case above, JavaScript encoding does not mitigate against DOM based XSS. In a DOM-based attacks, the HTTP response on the server side does not change. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. For example, you might need to close some existing elements before using your JavaScript payload. - owasp-CheatSheetSeries . Each encoder, Html, JavaScript and Url, must be configured separately. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. For example: Modern web applications are typically built using a number of third-party libraries and frameworks, which often provide additional functions and capabilities for developers. Output encoding here will prevent XSS, but it will break the intended functionality of the application. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. The majority of DOM XSS vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. There are two ways to do this. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. If you must, the following examples describe some approaches that do and do not work. This behavior was often implemented using a vulnerable hashchange event handler, similar to the following: As the hash is user controllable, an attacker could use this to inject an XSS vector into the $() selector sink. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Safe list ranges are specified as Unicode code charts, not languages. However, you may still find vulnerable code in the wild. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. Customization of the safe list only affects encoders sourced via DI. 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. 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. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Content Security Policy - An allowlist that prevents content being loaded. Output Encoding. 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. Read the entire Acunetix Web Application Vulnerability Report. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. The problem is that if companyName had the value "Johnson & Johnson". 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. In those cases, create a Trusted Type object yourself. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. 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. Variables should not be interpreted as code instead of text. Please note, it is always dangerous design to put untrusted data directly into a command execution context. 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. XSS is one of the most common and dangerous web vulnerabilities, and it is . Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. You should apply HTML attribute encoding to variables being placed in most HTML attributes. One example of an attribute which is thought to be safe is innerText. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. Developers should use the following prevention steps to avoid introducing XSS into their application. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. Avoid treating untrusted data as code or markup within JavaScript code. 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. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). This document only discusses JavaScript bugs which lead to XSS. Record your progression from Apprentice to Expert. There are two distinct groups of cross-site scripting. Semgrep rule to identify above dom xss link. This is a Safe Sink and will automatically URL encode data in it. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. How to detect DOM-based cross-site scripting?
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