dom based cross site scripting prevention

Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. 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). Output encoding is not perfect. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. DOM-based XSS Examples. Read more about DOM-based cross-site scripting. These types of attacks typically occur as a result . Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? Dangerous contexts include: Don't place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully. In a DOM-based attacks, the HTTP response on the server side does not change. DOM-based XSS is an advanced XSS attack. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. For example, websites often reflect URL parameters in the HTML response from the server. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. For example.. An attacker could modify data that is rendered as $varUnsafe. The most common one would be adding it to an href or src attribute of an tag. OWASP recommends DOMPurify for HTML Sanitization. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. 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. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. Never rely on validation alone. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. Read the entire Acunetix Web Application Vulnerability Report. 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. Testing JavaScript execution sinks for DOM-based XSS is a little harder. Misconceptions abound related to the proper encoding that is required. Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. This cheatsheet addresses DOM (Document Object Model) based XSS and is an extension (and assumes comprehension of) the XSS Prevention Cheatsheet. For example, the general rule is to HTML Attribute encode untrusted data (data from the database, HTTP request, user, back-end system, etc.) Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. //The following does NOT work because of the encoded ";". That said, you should also analyze the CSP violations, as these trigger when the non-conforming code is executed. For a comprehensive list, check out the DOMPurify allowlist. These locations are known as dangerous contexts. 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%>. However, you may still find vulnerable code in the wild. 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. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. Each encoder, Html, JavaScript and Url, must be configured separately. Variables should not be interpreted as code instead of text. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). For example, a JavaScript encoded string will execute even though it is JavaScript encoded. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. Accelerate penetration testing - find more bugs, more quickly. HTML attribute encoding is a superset of HTML encoding and encodes additional characters such as " and '. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Output encoding here will prevent XSS, but it will break the intended functionality of the application. Depending on the user input, use a suitable escaping technique like HTML escape, CSS escape, JavaScript escape, URL escape, etc. Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. Document Object Model (DOM) Based XSS. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. //The following does NOT work because of the encoded "(" and ")". From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. You need to work through each available source in turn, and test each one individually. Record your progression from Apprentice to Expert. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. The web application dynamically generates a web page that contains this untrusted data. This means you will need to use alternative elements like img or iframe. If this isn't possible, then ensure the data is JavaScript encoded. 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. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. XSS is one of the most common and dangerous web vulnerabilities, and it is . DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). 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. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. Developers should use the following prevention steps to avoid introducing XSS into their application. The complication is compounded by the differing meanings and treatment of encoded values within each subcontext (HTML, HTML attribute, URL, and CSS) within the execution context. 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 . There may be times you want to insert a value into JavaScript to process in your view. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). 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. \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');",