Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Save time/money. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. Always pass untrusted input as a query string value. 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. WAFs also dont address the root cause of an XSS vulnerability. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. 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. 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. 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. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. 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. 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. Trusted Types force you to process a value. 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. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Output Encoding is recommended when you need to safely display data exactly as a user typed it in. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Limit access to object properties when using object[x] accessors (Mike Samuel). The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. 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. Trusted Types work by locking down the following risky sink functions. This is common when you want users to be able to customize the look and feel of their webpages. 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). Validation can be a useful tool in limiting XSS attacks. DOM based XSS vulnerabilities therefore have to be prevented on the client side. 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. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Some pure DOM-based vulnerabilities are self-contained within a single page. 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. If these methods are provided with untrusted input, then an XSS vulnerability could result. 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. Based on this context, you need to refine your input to see how it is processed. The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. 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. 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. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. - owasp-CheatSheetSeries . We will look at eval, href and dangerouslySetHTML vulnerabilities. It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. What would be displayed in the input text field would be "Johnson & Johnson". Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. Use a CSP as an additional layer of defense and have a look at the. In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. CSS is surprisingly powerful and has been used for many types of attacks. In many cases the context isn't always straightforward to discern. 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. If you pollute a river, it'll flow downstream somewhere. Sometimes users need to author HTML. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. Output Encoding. The world's #1 web penetration testing toolkit. Variables should not be interpreted as code instead of text. If this isn't possible, then ensure the data is JavaScript encoded. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. The enterprise-enabled dynamic web vulnerability scanner. Output Encoding and HTML Sanitization help address those gaps. 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. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) 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. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. The line above could have possibly worked to render a link. This could lead to an attack being added to a webpage.. for example. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. Input validation. 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. Safe list ranges are specified as Unicode code charts, not languages. Get help and advice from our experts on all things Burp. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. 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. element.SetAttribute () element [attribute]= 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. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. 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 few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. 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. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. It also enables you to easily search your data without having to encode values before searching and allows you to take advantage of any changes or bug fixes made to encoders. Accelerate penetration testing - find more bugs, more quickly. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Validate all data that flows into your application from the server or a third-party API. Enhance security monitoring to comply with confidence. Developers should use the following prevention steps to avoid introducing XSS into their application. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". 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. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. 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. 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. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Browsers change functionality and bypasses are being discovered regularly. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. 99% of the time it is an indication of bad or lazy programming practice, so simply don't do it instead of trying to sanitize the input. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Use URL Encoding for these scenarios. The example that follows illustrates using closures to avoid double JavaScript encoding. 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. 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. 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. The data is subsequently read from the DOM by the web application and outputted to the browser. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. There may be times you want to insert a value into JavaScript to process in your view. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. DOM-based XSS Examples. The problem is that if companyName had the value "Johnson & Johnson". Encode all characters using the \xHH format. URL Contexts refer to variables placed into a URL. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Misconceptions abound related to the proper encoding that is required. 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. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. Read the entire Acunetix Web Application Vulnerability Report. See how our software enables the world to secure the web. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. Read the entire Acunetix Web Application Vulnerability Report. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. 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. Event handlers such as onload and onerror can be used in conjunction with these elements. Common injection vectors include document.url, document.location, and document.referrer objects. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. Please refer to the list below for details. XSS Prevention & Mitigation. Use only safe functions like document.innerText and document.textContent. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). Don't use untrusted input as part of a URL path. 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. 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. You need to work through each available source in turn, and test each one individually. Therefore, the primary recommendation is to avoid including untrusted data in this context. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. Testing JavaScript execution sinks for DOM-based XSS is a little harder. 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. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. For example.. An attacker could modify data that is rendered as $varUnsafe. These locations are known as dangerous contexts. . DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. In other words, add a level of indirection between untrusted input and specified object properties. 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. This cheat sheet provides guidance to prevent XSS vulnerabilities. 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. 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. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. Acunetix developers and tech agents regularly contribute to the blog. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. The best way to fix DOM based cross-site scripting is to use the right output method (sink). //The following DOES WORK because the encoded value is a valid variable name or function reference. 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. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. Each encoder, Html, JavaScript and Url, must be configured separately. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. 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 instance, jQuery's attr() function can change the attributes of DOM elements. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. 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. 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. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? 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. HTML tag elements are well defined and do not support alternate representations of the same tag. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. In DOM-based cross-site scripting, the HTML source code and response of the attack . 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. DOM XSS stands for Document Object Model-based Cross-site Scripting. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. OWASP recommends DOMPurify for HTML Sanitization. This is a Safe Sink and will automatically URL encode data in it. This is because the rule to HTML attribute encode in an HTML attribute rendering context is necessary in order to mitigate attacks which try to exit out of an HTML attributes or try to add additional attributes which could lead to XSS. 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. Get your questions answered in the User Forum. 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. 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. 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. *Encoder.Default then the default, Basic Latin only safelist will be used. . Safe HTML Attributes include: 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. 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. 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%>. This means you will need to use alternative elements like img or iframe. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. 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. Despite being rare, they may cause serious problems and only a few scanners can detect them. There are several methods and attributes which can be used to directly render HTML content within JavaScript. 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. 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. Otherwise, again, your security efforts are void. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. There are two distinct groups of cross-site scripting. On the client side, the HTTP response does not change but the script executes in malicious manner. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Trusted Types heavily reduce the DOM XSS attack surface of your application. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. 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. With Reflected/Stored the attack is injected into the application during server-side processing of requests where untrusted input is dynamically added to HTML. 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. Validation becomes more complicated when accepting HTML in user input. The DOM-based cross-site scripting requires the user to open an infected 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.

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