dom based cross site scripting preventiondom based cross site scripting prevention

Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. Browsers change functionality and bypasses are being discovered regularly. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. This cheat sheet provides guidance to prevent XSS vulnerabilities. What's the best way to prevent XSS attacks? | TechTarget DOM-based Cross-Site Scripting Attack in Depth - GeeksforGeeks Customization of the safe list only affects encoders sourced via DI. 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. 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. 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. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). 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. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. Make sure any attributes are fully quoted, same as JS and CSS. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. (It's free!). If A is double JavaScript encoded then the following if check will return false. 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 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. 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 innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. This can be done via a function such as: There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. DOM-based vulnerabilities occur in the content processing stage performed on the client, typically in client-side JavaScript. In those cases, create a Trusted Type object yourself. 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). Trusted Types require you to process the data before passing it to the above sink functions. If this isn't possible, then ensure the data is JavaScript encoded. For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS. 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. What's the difference between Pro and Enterprise Edition? If you need to render different content, use innerText instead of innerHTML. Catch critical bugs; ship more secure software, more quickly. See how our software enables the world to secure the web. Cross Site Scripting (XSS) | OWASP Foundation These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. Save time/money. XSS sinks are places where variables are placed into your webpage. 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. React XSS Guide: Examples and Prevention - StackHawk This means, that no data will be available in server logs. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. As with all other Cross-site Scripting (XSS) vulnerabilities, this type of attack also relies on insecure handling of user input on an HTML page. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). 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. 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. For each location where your string appears within the DOM, you need to identify the context. Before putting untrusted data inside an HTML element ensure it's HTML encoded. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. Some pure DOM-based vulnerabilities are self-contained within a single page. Consider adopting the following controls in addition to the above. 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. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. DOM XSS stands for Document Object Model-based Cross-site Scripting. 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. You need to work through each available source in turn, and test each one individually. 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. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. Get the latest content on web security in your inbox each week. XSS: What it is, how it works, and how to prevent it - Medium Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Please note, it is always dangerous design to put untrusted data directly into a command execution context. Already got an account? 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. The good news is that if user input is handled properly at the foundation level (e.g. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. //The following does NOT work because the event handler is being set to a string. Don't mutate DOM directly. For a comprehensive list, check out the DOMPurify allowlist. Trusted Types heavily reduce the DOM XSS attack surface of your application. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. In many cases the context isn't always straightforward to discern. See what Acunetix Premium can do for you. 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. Because the data was introduced in JavaScript code and passed to a URL subcontext the appropriate server-side encoding would be the following: Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following: There are a number of open source encoding libraries out there: Some work on a block list while others ignore important characters like "<" and ">". Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Output encoding is the primary defense against cross-site scripting vulnerabilities. The name originated from early versions of the attack where stealing data cross-site was the primary focus. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. There are two ways to do this. Each variable used in the user interface should be passed through an output encoding function. After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. 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. A Computer Science portal for geeks. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Your application can be vulnerable to both reflected/stored XSS and DOM XSS. A list of output encoding libraries is included in the appendix. 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. 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. Some examples of DOM-based XSS attacks include: 1. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Encoding libraries often have a EncodeForJavaScript or similar to support this function. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. Use a CSP as an additional layer of defense and have a look at the. If you're using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. Get started with Burp Suite Professional. A list of safe HTML attributes is provided in the Safe Sinks section. 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. 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. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. 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. Document Object Model (DOM) Based XSS. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. CSS Contexts refer to variables placed into inline CSS. \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');",