dom based cross site scripting prevention

DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. 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 these cases, HTML Sanitization should be used. Tag helpers will also encode input you use in tag parameters. This brings up an interesting design point. This is because these sinks treat the variable as text and will never execute it. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. There are two distinct groups of cross-site scripting. 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. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. The DOM, or Document Object Model, is the structural format used to . More info about Internet Explorer and Microsoft Edge. In many cases, JavaScript encoding does not stop attacks within an execution context. The DOM is a programming interface. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Document Object Model (DOM) Based XSS. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. 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. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. The name originated from early versions of the attack where stealing data cross-site was the primary focus. //any code passed into lName is now executable. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. 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. Reduce risk. This fact makes it more difficult to maintain web application security. On the client side, the HTTP response does not change but the script executes in malicious manner. A script within the later response contains a sink which then processes the data in an unsafe way. 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. Trusted Types force you to process a value. Automatic encoding and escaping functions are built into most frameworks. Catch critical bugs; ship more secure software, more quickly. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. Encode all characters with the %HH encoding format. 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. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. Please note, it is always dangerous design to put untrusted data directly into a command execution context. 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. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. However, you may still find vulnerable code in the wild. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. 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. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. 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. - owasp-CheatSheetSeries . Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. DOM XSS stands for Document Object Model-based Cross-site Scripting. //The following does NOT work because of the encoded "(" and ")". When your application no longer produces violations, you can start enforcing Trusted Types: Voila! For details, see the Google Developers Site Policies. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. 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. Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn't properly filter user input. Free, lightweight web application security scanning for CI/CD. 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. Developers should use the following prevention steps to avoid introducing XSS into their application. One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. 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. Read about other types of cross-site scripting attacks. How common is DOM-based cross-site scripting? You might find that the source gets assigned to other variables. 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. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. This variable includes some characters which are used in XSS attacks, namely <, " and >. For example. What's the difference between Pro and Enterprise Edition? 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. 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. A Computer Science portal for geeks. URL Contexts refer to variables placed into a URL. This means, that no data will be available in server logs. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. 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. Avoid treating untrusted data as code or markup within JavaScript code. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Sometimes you can't change the offending code. What would be displayed in the input text field would be "Johnson & Johnson". 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. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. For example.. An attacker could modify data that is rendered as $varUnsafe. 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. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. It is also impossible to protect against such client-side attacks using WAFs. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). 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 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%>. Event handlers such as onload and onerror can be used in conjunction with these elements. However, depending on the tag which innerText is applied, code can be executed. element.SetAttribute () element [attribute]= 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 ">". 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. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. To test for DOM XSS in an HTML sink, place a random alphanumeric string into the source (such as location.search), then use developer tools to inspect the HTML and find where your string appears. DOM-based XSS is a kind of XSS occurring entirely on the client-side. 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. . 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. The web application dynamically generates a web page that contains this untrusted data. Here are some examples of encoded values for specific characters. Acunetix developers and tech agents regularly contribute to the blog. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. 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. For example, when your application passes a string to innerHTML, the browser sends the following report: This says that in https://my.url.example/script.js on line 39 innerHTML was called with the string beginning with <img src=x. . The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. Use a CSP as an additional layer of defense and have a look at the. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. Trusted Types give you the tools to write, security review, and maintain applications free of DOM XSS vulnerabilities by making the dangerous web API functions secure by default. Doing so encourages designs in which the security rules are close to the data that they process, where you have the most context to correctly sanitize the value. //The following does NOT work because the event handler is being set to a string. The example that follows illustrates using closures to avoid double JavaScript encoding. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Practise exploiting vulnerabilities on realistic targets. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. Sometimes users need to author HTML. In other words, add a level of indirection between untrusted input and specified object properties. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Please refer to the list below for details. Validate all data that flows into your application from the server or a third-party API. Also, XSS attacks always execute in the browser. In the case above, JavaScript encoding does not mitigate against DOM based XSS. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. . Always pass untrusted input as a query string value. These methods constitute the HTML Subcontext within the Execution Context. 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. There are some further things to consider: Security professionals often talk in terms of sources and sinks. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. The OWASP Cheat Sheet Series was created to provide a concise collection of high value information on specific application security topics. Download the latest version of Burp Suite. 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. The document.write sink works with script elements, so you can use a simple payload, such as the one below: Note, however, that in some situations the content that is written to document.write includes some surrounding context that you need to take account of in your exploit. This is common when you want users to be able to customize the look and feel of their webpages.

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