Fewer XSS bugs appear in applications built with modern web frameworks. This means you will need to use alternative elements like img or iframe. The name originated from early versions of the attack where stealing data cross-site was the primary focus. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Limit access to object properties when using object[x] accessors (Mike Samuel). More info about Internet Explorer and Microsoft Edge. The best way to fix DOM based cross-site scripting is to use the right output method (sink). The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. Spaces, quotes, punctuation and other unsafe characters will be percent encoded to their hexadecimal value, for example a space character will become %20. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. For DOM XSS, the attack is injected into the application during runtime in the client directly. HTML Context refers to inserting a variable between two basic HTML tags like a
or . 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. If you directly access an encoder via System.Text.Encodings.Web. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. In these cases, HTML Sanitization should be used. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. Get started with Burp Suite Professional. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. This is why you would need to HTML encode too. 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. 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. While DOM-based XSS is a client-side injection vulnerability, the malicious payloads are executed by code originating from the server. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. There are a couple of options for fixing a Trusted Type violation. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Read the entire Acunetix Web Application Vulnerability Report. It is a simple yet effective way to harvest passwords using only the victims browser. However the opposite is the case with HTML encoding. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. 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 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. Sometimes you can't change the offending code. Parsing HTML input is difficult, if not impossible. 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. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side. 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. So XSS has already been around for a while. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". 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. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. Stored XSS is considered the most damaging type of XSS attack. Cross Site Scripting Prevention Cheat Sheet - OWASP There are several methods and attributes which can be used to directly render HTML content within JavaScript. 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. The enterprise-enabled dynamic web vulnerability scanner. Before putting untrusted data into a URL query string ensure it's URL encoded. If these methods are provided with untrusted input, then an XSS vulnerability could result. DOM based cross site scripting (Video solution) - YouTube 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). However, frameworks aren't perfect and security gaps still exist in popular frameworks like React and Angular. . 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. Prevent DOM-based cross-site scripting vulnerabilities with Trusted Types DOM-based Cross Site Scripting : DOM XSS stands for Document Object Model-based Cross-site Scripting. For example, websites often reflect URL parameters in the HTML response from the server. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. Cross Site Scripting Prevention Cheat Sheet - github.com Please insert your password to refresh your session. OWASP recommends DOMPurify for HTML Sanitization. WAFs also dont address the root cause of an XSS vulnerability. Some examples of DOM-based XSS attacks include: 1. How to detect DOM-based cross-site scripting? In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. There are two ways to do this. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. 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. The DOM, or Document Object Model, is the structural format used to . In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. Any variable that does not go through this process is a potential weakness. In this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. Consider adopting the following controls in addition to the above. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. Developers should use the following prevention steps to avoid introducing XSS into their application. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Get your questions answered in the User Forum. If this is the case, you'll need to use the search function again to track these variables and see if they're passed to a sink. 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. These types of attacks typically occur as a result . Safe list ranges are specified as Unicode code charts, not languages. URL Contexts refer to variables placed into a URL. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. 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. JavaScript Contexts refer to placing variables into inline JavaScript which is then embedded in an HTML document. //any code passed into lName is now executable. This is where Output Encoding and HTML Sanitization are critical. 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. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. The DOM is a programming interface. This view outputs the contents of the untrustedInput variable. DOM based XSS vulnerabilities therefore have to be prevented on the client side. 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 is a Safe Sink and will automatically CSS encode data in it. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. The DOM-based cross-site scripting requires the user to open an infected page. 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. DOMPurify supports Trusted Types and will return sanitized HTML wrapped in a TrustedHTML object such that the browser does not generate a violation.CautionIf the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. 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. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. 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. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. Catch critical bugs; ship more secure software, more quickly. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). If your code looked like the following, you would need to only double JavaScript encode input data. 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. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. 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. Output encoding is not perfect. 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. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. React XSS Guide: Examples and Prevention - StackHawk It is also impossible to protect against such client-side attacks using WAFs. placed in an HTML Attribute. Save time/money. 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. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. If A is double JavaScript encoded then the following if check will return false. 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. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output. For example. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. 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, this could be used by an attacker to subvert internal and external attributes of the myMapType object. The HTML encoded value above is still executable. Websites may also store data on the server and reflect it elsewhere. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. 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. // is an example of untrusted data that was properly JavaScript encoded but still executes. 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. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. I will show you three examples of DOM-based XSS attacks in this article. Already got an account? Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). 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. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). 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. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. However, you may still find vulnerable code in the wild. 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. The other alternative is using N-levels of encoding. Therefore, the primary recommendation is to avoid including untrusted data in this context. Event handlers such as onload and onerror can be used in conjunction with these elements. The #redir route is executed by another file, redir.html. In many cases, JavaScript encoding does not stop attacks within an execution context. Level up your hacking and earn more bug bounties. You might find that the source gets assigned to other variables. DOM based XSS Prevention Cheat Sheet - GitHub Learn the details here including XSS prevention methods. 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. 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. 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. 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 is a Safe Sink and will automatically URL encode data in it. However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. 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. 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. 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. Sometimes users need to author HTML. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Trusted Types require you to process the data before passing it to the above sink functions. What is Cross-Site Scripting (XSS) and How to Prevent It? Thankfully, many sinks where variables can be placed are safe. Other CSS Contexts are unsafe and you should not place variable data in them. 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. In these scenarios, you should do URL encoding, followed by HTML attribute encoding. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. OWASP TOP 10: Cross-site scripting (XSS) ~2023 | Udemy A list of output encoding libraries is included in the appendix. Despite being rare, they may cause serious problems and only a few scanners can detect them. If you must, the following examples describe some approaches that do and do not work. Based on this context, you need to refine your input to see how it is processed. Scale dynamic scanning. CSS Contexts refer to variables placed into inline CSS. Its critical to use quotation marks like " or ' to surround your variables. DOM-based XSS: DOM-based XSS occurs when an . Here are some examples of encoded values for specific characters. 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. As we use reCAPTCHA, you need to be able to access Google's servers to use this function. In the case above, JavaScript encoding does not mitigate against DOM based XSS. The only safe location for placing variables in JavaScript is inside a quoted data value. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. Avoid methods such as document.innerHTML and instead use safer functions, for example, document.innerText and document.textContent. Avoid treating untrusted data as code or markup within JavaScript code. eval Directly setting event handler attributes will allow JavaScript encoding to mitigate against DOM based XSS. 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. What would be displayed in the input text field would be "Johnson & Johnson". Browsers change functionality and bypasses are being discovered regularly. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. 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. Each encoder, Html, JavaScript and Url, must be configured separately. 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. . . OWASP recommends these in all circumstances. You must ensure that you only use @ in an HTML context, not when attempting to insert untrusted input directly into JavaScript. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. What is Cross-Site Scripting (XSS)? How to Prevent it? | Fortinet The name originated from early versions of the attack where stealing data cross-site was the primary focus. 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. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. Use only safe functions like document.innerText and document.textContent. For example, you might need to close some existing elements before using your JavaScript payload. //The following does NOT work because the event handler is being set to a string. Always pass untrusted input as a query string value. DOM-based XSS is a kind of XSS occurring entirely on the client-side. Practise exploiting vulnerabilities on realistic targets. Get the latest content on web security in your inbox each week. Avoid populating the following methods with untrusted data. This can be done via a function such as: Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. 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. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost.