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// is an example of untrusted data that was properly JavaScript encoded but still executes. Output Encoding and HTML Sanitization help address those gaps. . Output Encoding. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. 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. Semgrep rule to identify above dom xss link. On the client side, the HTTP response does not change but the script executes in malicious manner. In a DOM-based attacks, the HTTP response on the server side does not change. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. If you pollute a river, it'll flow downstream somewhere. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. Encode all characters using the \xHH format. Event handlers such as onload and onerror can be used in conjunction with these elements. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. Output encoding is not perfect. Sometimes you can't change the offending code. 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. Enhance security monitoring to comply with confidence. Read about other types of cross-site scripting attacks. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. When a browser is rendering HTML and any other associated content like CSS or JavaScript, it identifies various rendering contexts for the different kinds of input and follows different rules for each context. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). It uses HTML attribute encoding rules whenever you use the @ directive. 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. 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). 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. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. 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. XSS sinks are places where variables are placed into your webpage. Before putting untrusted data into a URL query string ensure it's URL encoded. 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. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. There may be times you want to insert a value into JavaScript to process in your view. 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(). To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. Trusted Types heavily reduce the DOM XSS attack surface of your application. If these methods are provided with untrusted input, then an XSS vulnerability could result. For details, see the Google Developers Site Policies. 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. OWASP recommends these in all circumstances. Never rely on validation alone. One example of an attribute which is thought to be safe is innerText. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. For that, first create a policy. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. 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. I will show you three examples of DOM-based XSS attacks in this article. 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. What would be displayed in the input text field would be "Johnson & Johnson". Each variable in a web application needs to be protected. Get started with Burp Suite Enterprise Edition. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. 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. Some pure DOM-based vulnerabilities are self-contained within a single page. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. XSS Prevention & Mitigation. Developers should use the following prevention steps to avoid introducing XSS into their application. Acunetix developers and tech agents regularly contribute to the blog. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. Parsing HTML input is difficult, if not impossible. Use only safe functions like document.innerText and document.textContent. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. This is why you would need to HTML encode too. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. These types of attacks typically occur as a result . 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. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. 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). There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Common injection vectors include document.url, document.location, and document.referrer objects. For example. In other words, add a level of indirection between untrusted input and specified object properties. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. Each variable used in the user interface should be passed through an output encoding function. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. How to detect DOM-based cross-site scripting? You might find that the source gets assigned to other variables. Variables should not be interpreted as code instead of text. Sometimes users need to author HTML. The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. Tag helpers will also encode input you use in tag parameters. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". (It's free!). This helps quickly identify a large chunk of violations. DOM-based XSS: DOM-based XSS occurs when an . 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 your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. The name originated from early versions of the attack where stealing data cross-site was the primary focus. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". The #redir route is executed by another file, redir.html. The logic which parses URLs in both execution and rendering contexts looks to be the same. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. This is where Output Encoding and HTML Sanitization are critical. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. 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. The DOM is a programming interface. If this isn't possible, then ensure the data is JavaScript encoded. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. You need to work through each available source in turn, and test each one individually. Stored XSS is considered the most damaging type of XSS attack. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. However the opposite is the case with HTML encoding. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. For each location where your string appears within the DOM, you need to identify the context. 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%>. 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. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. Download the latest version of Burp Suite. The only safe location for placing variables in JavaScript is inside a quoted data value. . It will not always prevent XSS. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. 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. 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. //The following does NOT work because the event handler is being set to a string. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. DOM-based cross-site scripting happens when data from a user controlled, Most of the violations like this can also be detected by running a code linter or, If the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. The world's #1 web penetration testing toolkit. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. 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. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. The other alternative is using N-levels of encoding. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. This cheat sheet provides guidance to prevent XSS vulnerabilities. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. 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. 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. This is a Safe Sink and will automatically CSS encode data in it. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. We want to hear from you! The rendered output would now become. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Read more about DOM-based cross-site scripting. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Reduce risk. Validation can be a useful tool in limiting XSS attacks. Please refer to the list below for details. Cross-Site Scripting (XSS) is a misnomer. A list of safe HTML attributes is provided in the Safe Sinks section. - owasp-CheatSheetSeries . This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The styling will not be rendered.