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. The HTML parser of the rendering context dictates how data is presented and laid out on the page and can be further broken down into the standard contexts of HTML, HTML attribute, URL, and CSS. Testing JavaScript execution sinks for DOM-based XSS is a little harder. The web application dynamically generates a web page that contains this untrusted data. 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. However the opposite is the case with HTML encoding. Tag helpers will also encode input you use in tag parameters. 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. element.SetAttribute () element [attribute]= For example, this is the case if you're loading a third-party library from a CDN. 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. Catch critical bugs; ship more secure software, more quickly. The HTML encoded value above is still executable. Any variable that does not go through this process is a potential weakness. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. The most common one would be adding it to an href or src attribute of an tag. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to customFunction. DOM-based XSS Examples. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. 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 following charts details a list of critical output encoding methods needed to stop Cross Site Scripting. 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. In the case above, JavaScript encoding does not mitigate against DOM based XSS. 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. Limit access to object properties when using object[x] accessors (Mike Samuel). Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Also, XSS attacks always execute in the browser. Different sources and sinks have various properties and behaviors that can impact exploitability, and determine what methods are used. These locations are known as dangerous contexts. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. Output encoding is the primary defense against cross-site scripting vulnerabilities. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. 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). Dangerous attributes include any attribute that is a command execution context, such as onclick or onblur. Output Encoding. This fact makes it more difficult to maintain web application security. Use a trusted and verified library to escape HTML inputs. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. If you directly access an encoder via System.Text.Encodings.Web. Event handlers such as onload and onerror can be used in conjunction with these elements. 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. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. 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. In Chrome's developer tools, you can use Control+F (or Command+F on MacOS) to search the DOM for your string. Now a browser can also help prevent the client-side (also known as DOM-based) XSSes with Trusted Types. . Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. In some . This can be done via a function such as: Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. (It's free!). Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. 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. Enhance security monitoring to comply with confidence. 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(). : You can customize the encoder safe lists to include Unicode ranges appropriate to your application during startup, in ConfigureServices(). DOM Based Attacks. The rendered output would now become. Get your questions answered in the User Forum. When this happens, a script on the web page selects the URL variable and executes the code it contains. 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. However, if the pages returned from your web application utilize a content type of text/xhtml or the file type extension of *.xhtml then HTML encoding may not work to mitigate against XSS. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. Variables should not be interpreted as code instead of text. This means you will need to use alternative elements like img or iframe. When a site uses the ng-app attribute on an HTML element, it will be processed by AngularJS. The primary difference is where the attack is injected into the application. Prevent XSS by sanitizing user data on the backend, HTML-encode user-provided data that's rendered into the template, and . Automatic encoding and escaping functions are built into most frameworks. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value. 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. A Computer Science portal for geeks. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). This is where Output Encoding and HTML Sanitization are critical. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. DOM-based XSS is a kind of XSS occurring entirely on the client-side. No single technique will solve XSS. 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. This document only discusses JavaScript bugs which lead to XSS. 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. Note that browsers behave differently with regards to URL-encoding, Chrome, Firefox, and Safari will URL-encode location.search and location.hash, while IE11 and Microsoft Edge (pre-Chromium) will not URL-encode these sources. 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. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. If you're using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). 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 (#). 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. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. DOM XSS stands for Document Object Model-based Cross-site Scripting. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. It is almost impossible to detect DOM XSS only from the server-side (using HTTP requests). If you pollute a river, it'll flow downstream somewhere. We will look at eval, href and dangerouslySetHTML vulnerabilities. The best way to fix DOM based cross-site scripting is to use the right output method (sink). There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. If this isn't possible, then ensure the data is JavaScript encoded. There are a variety of sinks that are relevant to DOM-based vulnerabilities. Validate all data that flows into your application from the server or a third-party API. //The following DOES WORK because the encoded value is a valid variable name or function reference. For example, Acunetix. 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. This behavior also affects Razor TagHelper and HtmlHelper rendering as it will use the encoders to output your strings. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. Variables should only be placed in a CSS property value. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Some XSS vulnerabilities are caused by the server-side code that insecurely creates the HTML code forming the website. In a reflected DOM XSS vulnerability, the server processes data from the request, and echoes the data into the response. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. 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. Scale dynamic scanning. However, this could be used by an attacker to subvert internal and external attributes of the myMapType object. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. 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. 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. The setAttribute(name_string,value_string) method is dangerous because it implicitly coerces the value_string into the DOM attribute datatype of name_string. Accelerate penetration testing - find more bugs, more quickly. 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. \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');",