dom based cross site scripting prevention

In reflective and stored cross-site scripting attacks, you can see the vulnerability payload in the response page. 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. If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. For example: To make dynamic updates to HTML in the DOM safe, we recommend: The HTML attribute subcontext within the execution context is divergent from the standard encoding rules. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Each parser has distinct and separate semantics in the way they can possibly execute script code which make creating consistent rules for mitigating vulnerabilities in various contexts difficult. These attacks belong to the subset of client cross-site scripting as the data source is from the client side only. In DOM-based cross-site scripting, the HTML source code and response of the attack . This cheat sheet provides guidance to prevent XSS vulnerabilities. Variables should not be interpreted as code instead of text. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. This is a Safe Sink and will automatically URL encode data in it. 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. DOM-based XSS is a kind of XSS occurring entirely on the client-side. Get the latest content on web security in your inbox each week. 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. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. Do your applications use this vulnerable package? Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. (It's free!). After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. There are some further things to consider: Security professionals often talk in terms of sources and sinks. 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. document.CreateTextNode () and append it in the appropriate DOM location. DOM XSS stands for Document Object Model-based Cross-site Scripting. This is a Safe Sink and will automatically CSS encode data in it. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. 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. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. 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. This is because these sinks treat the variable as text and will never execute it. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Its the same with computer security. 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. A script within the later response contains a sink which then processes the data in an unsafe way. 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. Use a trusted and verified library to escape HTML inputs. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. You need to work through each available source in turn, and test each one individually. 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. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. Therefore, the primary recommendation is to avoid including untrusted data in this context. It will not always prevent XSS. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. 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. 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. Read the entire Acunetix Web Application Vulnerability Report. Trusted Types require you to process the data before passing it to the above sink functions. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. For example, websites often reflect URL parameters in the HTML response from the server. Read the entire Acunetix Web Application Vulnerability Report. Validate all data that flows into your application from the server or a third-party API. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. HTML tag elements are well defined and do not support alternate representations of the same tag. Acunetix Web Application Vulnerability Report 2020, How To Prevent DOM-based Cross-site Scripting, DOM XSS: An Explanation of DOM-based Cross-site Scripting, Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS, Finding the Source of a DOM-based XSS Vulnerability with Acunetix, Read about other types of cross-site scripting attacks. Cross-Site Scripting (XSS) is a misnomer. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. You might find that the source gets assigned to other variables. We are looking for web developers to participate in user research, product testing, discussion groups and more. 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 practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Trusted Types force you to process a value. When other users load affected pages the attacker's scripts will run, enabling the attacker to steal cookies and session tokens, change the contents of the web page through DOM manipulation or redirect the browser to another page. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. 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. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. If you sanitize content and then modify it afterwards, you can easily void your security efforts. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. Get your questions answered in the User Forum. WAFs are unreliable and new bypass techniques are being discovered regularly. What would be displayed in the input text field would be "Johnson & Johnson". Sometimes users need to author HTML. One example of an attribute which is thought to be safe is innerText. Don't mutate DOM directly. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. 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%>. These locations are known as dangerous contexts. . Before putting untrusted data into a URL query string ensure it's URL encoded. In many cases, JavaScript encoding does not stop attacks within an execution context. In other words, add a level of indirection between untrusted input and specified object properties. Practise exploiting vulnerabilities on realistic targets. 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. 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. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. 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. element.SetAttribute () element [attribute]= The encoder safe lists can be customized to include Unicode ranges appropriate to the app during startup, in Program.cs: For example, using the default configuration using a Razor HtmlHelper similar to the following: The preceding markup is rendered with Chinese text encoded: To widen the characters treated as safe by the encoder, insert the following line into Program.cs. Variables should only be placed in a CSS property value. 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. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. For example.. An attacker could modify data that is rendered as $varUnsafe. eval Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. 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. 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). Rather, a malicious change in the DOM environment causes client code to run unexpectedly. 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=. 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. The data is subsequently read from the DOM by the web application and outputted to the browser. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Browsers change functionality and bypasses are being discovered regularly. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. 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. Document Object Model (DOM) Based XSS. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. DOM Based Attacks. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Tag helpers will also encode input you use in tag parameters. The Unicode standard has a list of code charts you can use to find the chart containing your characters. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). Use URL Encoding for these scenarios. 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. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. It is also impossible to protect against such client-side attacks using WAFs. Content Security Policy - An allowlist that prevents content being loaded. 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. - owasp-CheatSheetSeries . 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. The safest way to insert values is to place the value in a data attribute of a tag and retrieve it in your JavaScript. 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. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. Learn more about types of cross-site scripting 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. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. The DOM is a programming interface. An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. The following are some of the main sinks that can lead to DOM-XSS vulnerabilities: The following jQuery functions are also sinks that can lead to DOM-XSS vulnerabilities: In addition to the general measures described on the DOM-based vulnerabilities page, you should avoid allowing data from any untrusted source to be dynamically written to the HTML document. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. However, depending on the tag which innerText is applied, code can be executed. 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. Get the latest content on web security in your inbox each week. 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. This brings up an interesting design point. *Encoder.Default then the default, Basic Latin only safelist will be used. 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. If you're using JavaScript to change a CSS property, look into using style.property = x. CSS Contexts refer to variables placed into inline CSS. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. Web Application Firewalls - These look for known attack strings and block them. 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. Read about other types of cross-site scripting attacks. Please refer to the list below for details. 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. When this happens, a script on the web page selects the URL variable and executes the code it contains. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. Already got an account? 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). These methods constitute the HTML Subcontext within the Execution Context. 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. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Reduce risk. See what Acunetix Premium can do for you. The payload can be manipulated to deface the target application using a prompt that states: Your session has expired. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. 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. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. For each location where your string appears within the DOM, you need to identify the context. 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. 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. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. In some . DOM-based XSS: DOM-based XSS occurs when an . It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. 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. Want to track your progress and have a more personalized learning experience? 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. However, you may still find vulnerable code in the wild. For example: Modern web applications are typically built using a number of third-party libraries and frameworks, which often provide additional functions and capabilities for developers. \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');",