Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. your framework), you should be able to mitigate all XSS vulnerabilities. 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). If this isn't possible, then ensure the data is JavaScript encoded. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Get help and advice from our experts on all things Burp. This means, that no data will be available in server logs. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. 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. 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 could lead to an attack being added to a webpage.. for example. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. Get your questions answered in the User Forum. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. 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. 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. 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. Avoid populating the following methods with untrusted data. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. OWASP recommends DOMPurify for HTML Sanitization. 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. The line above could have possibly worked to render a link. If your code looked like the following, you would need to only double JavaScript encode input data. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. HTML Context refers to inserting a variable between two basic HTML tags like a
or . 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. 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. 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. These methods constitute the HTML Subcontext within the Execution Context. Thankfully, many sinks where variables can be placed are safe. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Learn the details here including XSS prevention methods. 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. 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. 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 script within the later response contains a sink which then processes the data in an unsafe way. Output Encoding and HTML Sanitization help address those gaps. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Some examples of DOM-based XSS attacks include: 1. 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. However, if the pages returned from your web application utilize a content type of text/xhtml or the file type extension of *.xhtml then HTML encoding may not work to mitigate against XSS. 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. Download the latest version of Burp Suite. How to prevent DOM-based cross-site scripting? However, you may still find vulnerable code in the wild. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. - owasp-CheatSheetSeries . Variables should not be interpreted as code instead of text. This is why you would need to HTML encode too. Acunetix developers and tech agents regularly contribute to the blog. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Don't use untrusted input as part of a URL path. 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. Input validation. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. Event handlers such as onload and onerror can be used in conjunction with these elements. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. 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. Otherwise, again, your security efforts are void. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. That said, developers need to be aware of problems that can occur when using frameworks insecurely such as: Understand how your framework prevents XSS and where it has gaps. You should apply HTML attribute encoding to variables being placed in most HTML attributes. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. All other contexts are unsafe and you should not place variable data in them. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. If you pollute a river, it'll flow downstream somewhere. The web application dynamically generates a web page that contains this untrusted data. Use only safe functions like document.innerText and document.textContent. 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. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. 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. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Do your applications use this vulnerable package? The styling will not be rendered. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. For instance, jQuery's attr() function can change the attributes of DOM elements. 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. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. 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. Document Object Model (DOM) Based XSS. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! 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. For example.. An attacker could modify data that is rendered as $varUnsafe. //The following DOES WORK because the encoded value is a valid variable name or function reference. 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. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. 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. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Use a CSP as an additional layer of defense and have a look at the. Please note, element.setAttribute is only safe for a limited number of attributes. 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. Despite being rare, they may cause serious problems and only a few scanners can detect them. . It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. 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. 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. Any variable that does not go through this process is a potential weakness. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. document.CreateTextNode () and append it in the appropriate DOM location. 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%>. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Some pure DOM-based vulnerabilities are self-contained within a single page. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. 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. Before putting untrusted data into a URL query string ensure it's URL encoded. DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. XSS Prevention & Mitigation. Canonicalize input, URL Validation, Safe URL verification, Allow-list http and HTTPS URLs only (Avoid the JavaScript Protocol to Open a new Window), Attribute encoder. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. *Encoder.Default then the default, Basic Latin only safelist will be used. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. No single technique will solve XSS. It will not always prevent XSS. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. 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. 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. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. This view outputs the contents of the untrustedInput variable. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. 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. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. 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 cheatsheet is a list of techniques to prevent or limit the impact of XSS. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. 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. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. 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. The attack functions by manipulating the internal model of the webpage within the browser known as the DOM and are referred to as DOM based attacks . Other CSS Contexts are unsafe and you should not place variable data in them. Browsers change functionality and bypasses are being discovered regularly. Make sure any attributes are fully quoted, same as JS and CSS. If A is double JavaScript encoded then the following if check will return false. In other words, add a level of indirection between untrusted input and specified object properties. Scale dynamic scanning. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. //any code passed into lName is now executable. Trusted Types force you to process a value somehow, but don't yet define what the exact processing rules are, and whether they are safe. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. 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. (It's free!). Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. The only safe location for placing variables in JavaScript is inside a quoted data value. 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. 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. This can be done via a function such as: ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. This is common when you want users to be able to customize the look and feel of their webpages. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. 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. 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. Websites may also store data on the server and reflect it elsewhere. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). Encode all characters using the \xHH format. The world's #1 web penetration testing toolkit. CSS is surprisingly powerful and has been used for many types of attacks. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. The DOM, or Document Object Model, is the structural format used to . HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. A list of safe HTML attributes is provided in the Safe Sinks section. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. How common is DOM-based cross-site scripting? HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. //The following does NOT work because the event handler is being set to a string. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. A DOM-based XSS attack is possible if the web application writes data to the DOM without proper sanitization. What's the difference between Pro and Enterprise Edition? Therefore, the primary recommendation is to avoid including untrusted data in this context. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). Note how the payload is stored in the GET request, making it suitable for social engineering attacks. Stored XSS is considered the most damaging type of XSS attack. //The following does NOT work because of the encoded "(" and ")". 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(). In many cases, JavaScript encoding does not stop attacks within an execution context. 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. Limit access to object properties when using object[x] accessors (Mike Samuel). DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). Already got an account? When this happens, a script on the web page selects the URL variable and executes the code it contains. Developers should use the following prevention steps to avoid introducing XSS into their application. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Tag helpers will also encode input you use in tag parameters. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. There are 3 primary types of cross-site scripting: DOM-based XSS. Want to track your progress and have a more personalized learning experience? Your application can be vulnerable to both reflected/stored XSS and DOM XSS. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. 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. Testing JavaScript execution sinks for DOM-based XSS is a little harder. 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. Get the latest content on web security in your inbox each week. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. 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. Get the latest content on web security in your inbox each week. Encode all characters with the %HH encoding format. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. DOM-based cross-site scripting (DOM XSS) is a web vulnerability, a subtype of cross-site scripting. However the opposite is the case with HTML encoding. 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. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). Trusted Types heavily reduce the DOM XSS attack surface of your application. In some . As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. DOM-Based Cross-Site Scripting. There are two ways to do this. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. If you need to render different content, use innerText instead of innerHTML. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Encoding libraries often have a EncodeForJavaScript or similar to support this function. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context.
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