For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. Read the entire Acunetix Web Application Vulnerability Report. These methods constitute the HTML Subcontext within the Execution Context. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. In other words, add a level of indirection between untrusted input and specified object properties. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. Trusted Types work by locking down the following risky sink functions. 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. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. \u0074\u0065\u0073\u0074\u0049\u0074\u003b\u0074\u0065\u0073. Misconceptions abound related to the proper encoding that is required. CSS Contexts refer to variables placed into inline CSS. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Read more about DOM-based cross-site scripting. Common injection vectors include document.url, document.location, and document.referrer objects. 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. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. Read more about DOM-based cross-site scripting. Use URL Encoding for these scenarios. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. The HTML encoded value above is still executable. For example, you might need to close some existing elements before using your JavaScript payload. The primary rule that you must follow to prevent DOM XSS is: sanitize all untrusted data, even if it is only used in client-side scripts. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. In many cases, JavaScript encoding does not stop attacks within an execution context. 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. Script manipulation: <script src> and setting text content of <script> elements. 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. When this happens, a script on the web page selects the URL variable and executes the code it contains. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. //any code passed into lName is now executable. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. There will be situations where you use a URL in different contexts. //The following does NOT work because of the encoded "(" and ")". This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. 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. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. See what Acunetix Premium can do for you. Also, XSS attacks always execute in the browser. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. Make sure any attributes are fully quoted, same as JS and CSS. What would be displayed in the input text field would be "Johnson & Johnson". 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. Enhance security monitoring to comply with confidence. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. This cheat sheet provides guidance to prevent XSS vulnerabilities. Encoding libraries often have a EncodeForJavaScript or similar to support this function. . Download the latest version of Burp Suite. 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. Browsers change functionality and bypasses are being discovered regularly. DOM XSS stands for Document Object Model-based Cross-site Scripting. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. The DOM is a programming interface. In many cases the context isn't always straightforward to discern. 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. DOM-based XSS is an advanced XSS attack. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. The other alternative is using N-levels of encoding. This variable includes some characters which are used in XSS attacks, namely <, " and >. Sometimes users need to author HTML. 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. Output encoding is not perfect. 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. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. It is the process of converting untrusted . There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. The following is an example vulnerability which occurs in the JavaScript context and HTML subcontext: Let's look at the individual subcontexts of the execution context in turn. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. One example of an attribute which is thought to be safe is innerText. There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). Get your questions answered in the User Forum. 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. Summary. To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript. 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. 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. For example, Acunetix. The best way to fix DOM based cross-site scripting is to use the right output method (sink). If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. Get the latest content on web security in your inbox each week. This document only discusses JavaScript bugs which lead to XSS. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. WAFs are unreliable and new bypass techniques are being discovered regularly. 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. For a comprehensive list, check out the DOMPurify allowlist. You need to work through each available source in turn, and test each one individually. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. 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. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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. Reduce risk. Parsing HTML input is difficult, if not impossible. DOM-Based Cross-Site Scripting. Customization of the safe list only affects encoders sourced via DI. //The following does NOT work because the event handler is being set to a string. If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. . There will be times where you need to do something outside the protection provided by your framework. 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. A rendering context is associated with the parsing of HTML tags and their attributes. Fewer XSS bugs appear in applications built with modern web frameworks. I will show you three examples of DOM-based XSS attacks in this article. For example.. An attacker could modify data that is rendered as $varUnsafe. 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. This is a Safe Sink and will automatically CSS encode data in it. Input validation. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. 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. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. In some . There are numerous methods which implicitly eval() data passed to it that must be avoided. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. 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. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. There may be times you want to insert a value into JavaScript to process in your view. We will look at eval, href and dangerouslySetHTML vulnerabilities. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. Stored XSS is considered the most damaging type of XSS attack. The data is subsequently read from the DOM by the web application and outputted to the browser. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. - owasp-CheatSheetSeries . For example, if your string appears within a double-quoted attribute then try to inject double quotes in your string to see if you can break out of the attribute. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Login here. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. However the opposite is the case with HTML encoding. It uses HTML attribute encoding rules whenever you use the @ directive. This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against XSS. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. We are looking for web developers to participate in user research, product testing, discussion groups and more. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. 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 . Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. 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. . These locations are known as dangerous contexts. This fact makes it more difficult to maintain web application security. 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. Variables should only be placed in a CSS property value. . Get started with Burp Suite Enterprise Edition. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. 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. 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. Then client-side encode (using a JavaScript encoding library such as node-esapi) for the individual subcontext (DOM methods) which untrusted data is passed to. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: createElement () and assign property values with appropriate methods or properties such as node.textContent= or node.InnerText=. However, depending on the tag which innerText is applied, code can be executed. 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. 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. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. DOM-based XSS: DOM-based XSS occurs when an . //The following does NOT work because of the encoded ";". Cookie Attributes - These change how JavaScript and browsers can interact with cookies. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Accelerate penetration testing - find more bugs, more quickly. This is where Output Encoding and HTML Sanitization are critical. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Sometimes you can't change the offending code. document.CreateTextNode () and append it in the appropriate DOM location. 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. It is also impossible to protect against such client-side attacks using WAFs. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. 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. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. View the source code of this file and note the following JavaScript code snippet: Essentially, the exploit uses the window.location.hash source, which is evaluated in an HTML element sink. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. DOM-based XSS is a kind of XSS occurring entirely on the client-side. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. 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. You might find that the source gets assigned to other variables. Validation becomes more complicated when accepting HTML in user input. Do your applications use this vulnerable package? For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. The web application dynamically generates a web page that contains this untrusted data. The primary difference is where the attack is injected into the application. 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. In those cases, create a Trusted Type object yourself. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement. Validate all data that flows into your application from the server or a third-party API. 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. your framework), you should be able to mitigate all XSS vulnerabilities. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. 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. A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. Please refer to the list below for details. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). For details, see the Google Developers Site Policies. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Output encoding here will prevent XSS, but it will break the intended functionality of the application. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. \u0064\u006f\u0063\u0075\u006d\u0065\u006e\u0074, \u0077\u0072\u0069\u0074\u0065\u006c\u006e, "\u0048\u0065\u006c\u006c\u006f\u0020\u0057\u006f\u0072\u006c\u0064", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0031\u0029", "url(<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(companyName))%>)", '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(userRelativePath))%>', "<%= Encode.forJavaScript(untrustedData) %>", "<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>", "customFunction('<%=doubleJavaScriptEncodedData%>', y)", //HTML encoding is happening in JavaScript, "javascript:myFunction('<%=untrustedData%>', 'test');", "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(untrustedData)) %>', 'test');", , "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. XSS sinks are places where variables are placed into your webpage. Ensure JavaScript variables are quoted, JavaScript Hex Encoding, JavaScript Unicode Encoding, Avoid backslash encoding (. Cookie attributes try to limit the impact of an XSS attack but dont prevent the execution of malicious content or address the root cause of the vulnerability. Never put untrusted data into your HTML input, unless you follow the rest of the steps below.
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