This could lead to an attack being added to a webpage.. for example. This fact makes it more difficult to maintain web application security. Acunetix developers and tech agents regularly contribute to the blog. Get started with Burp Suite Enterprise Edition. You might find that the source gets assigned to other variables. 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. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. Read more about DOM-based cross-site scripting. 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. The document.write sink works with script elements, so you can use a simple payload, such as the one below: Note, however, that in some situations the content that is written to document.write includes some surrounding context that you need to take account of in your exploit. 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. The best manual tools to start web security testing. : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). The good news is that if user input is handled properly at the foundation level (e.g. "\u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0029". After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. - owasp-CheatSheetSeries . How common is DOM-based cross-site scripting? Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. It is the process of converting untrusted . A DOM-based XSS attack is possible if the web application writes data to the Document Object Model without proper sanitization. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. Output encoding is the primary defense against cross-site scripting vulnerabilities. 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. It's important to remember that some of these are also potential sources and sinks for DOM XSS. 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. The best way to fix DOM based cross-site scripting is to use the right output method (sink). In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. How to prevent DOM-based cross-site scripting? JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. 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. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. 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%>. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. For DOM XSS, the attack is injected into the application during runtime in the client directly. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. 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. Here are the proper security techniques to use to prevent XSS attacks: Sanitize outputs properly. 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. Instead use JSON.toJSON() and JSON.parse() (Chris Schmidt). 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. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. XSS is one of the most common and dangerous web vulnerabilities, and it is . Get the latest content on web security in your inbox each week. 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 this case, AngularJS will execute JavaScript inside double curly braces that can occur directly in HTML or inside attributes. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. For example, this is the case if you're loading a third-party library from a CDN. 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. placed in an HTML Attribute. Enhance security monitoring to comply with confidence. This is because these sinks treat the variable as text and will never execute it. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. 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. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. It is also impossible to protect against such client-side attacks using WAFs. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). For more details on how to prevent DOM-based XSS attacks, you can read the OWASP DOM-based XSS Prevention Cheat Sheet. If you have to use user input on your page, always use it in the text context, never as HTML tags or any other potential code. It allows an attacker to circumvent the same origin policy, which is designed to segregate different websites from each other. If you have to use user input on your page, always use it in the text context, never as HTML tags or any other potential code. CSS is surprisingly powerful and has been used for many types of attacks. element.SetAttribute () element [attribute]= jQuery used to be extremely popular, and a classic DOM XSS vulnerability was caused by websites using this selector in conjunction with the location.hash source for animations or auto-scrolling to a particular element on the page. The example that follows illustrates using closures to avoid double JavaScript encoding. Summary. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". A Computer Science portal for geeks. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. 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. A script within the later response contains a sink which then processes the data in an unsafe way. Practise exploiting vulnerabilities on realistic targets. These types of attacks typically occur as a result . 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. The web application dynamically generates a web page that contains this untrusted data. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? 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. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. 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. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. 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. 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. It is a simple yet effective way to harvest passwords using only the victims browser. Always pass untrusted input as a query string value. Learn the details here including XSS prevention methods. For that, first create a policy. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Document Object Model (DOM) Based XSS. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. Semgrep rule to identify above dom xss link. Cross-site scripting (XSS) is a web security issue that sees cyber criminals execute malicious scripts on legitimate or trusted websites. A list of output encoding libraries is included in the appendix. 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. Scale dynamic scanning. 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. Make sure any attributes are fully quoted, same as JS and CSS. 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. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. There are numerous methods which implicitly eval() data passed to it that must be avoided. Output Encoding and HTML Sanitization help address those gaps. 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. Never rely on validation alone. 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. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. 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. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. Websites may also store data on the server and reflect it elsewhere. If this isn't possible, then ensure the data is JavaScript encoded. The enterprise-enabled dynamic web vulnerability scanner. How to find and test for XSS vulnerabilities You can use web vulnerability scanners to quickly find out XSS vulnerabilities. This cushions your application against an XSS attack, and at times, you may be able to prevent it, as well. The Unicode standard has a list of code charts you can use to find the chart containing your characters. 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. You should apply HTML attribute encoding to variables being placed in most HTML attributes. 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. Using the right combination of defensive techniques is necessary to prevent XSS. Otherwise, again, your security efforts are void. Perhaps the non-conforming functionality is not needed anymore or can be rewritten in a modern way without using the error-prone functions?Don'tel.innerHTML = '<img src=xyz.jpg>'; Doel.textContent = '';const img = document.createElement('img');img.src = 'xyz.jpg';el.appendChild(img); Some libraries already generate Trusted Types that you can pass to the sink functions. Based on this context, you need to refine your input to see how it is processed. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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. Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. Once you've found where the source is being read, you can use the JavaScript debugger to add a break point and follow how the source's value is used. HTML tag elements are well defined and do not support alternate representations of the same tag. Examining the source shows the rendered output encoded as: ASP.NET Core MVC provides an HtmlString class which isn't automatically encoded upon output.
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