placed in an HTML Attribute. You should apply HTML attribute encoding to variables being placed in most HTML attributes. More info about Internet Explorer and Microsoft Edge. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. There are some further things to consider: Security professionals often talk in terms of sources and sinks. Scale dynamic scanning. 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. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Its critical to use quotation marks like " or ' to surround your variables. Types of XSS attacks since mid-2012: DOM-based XSS attacks in React. One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script tag. 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 general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. When the iframe is loaded, an XSS vector is appended to the hash, causing the hashchange event to fire. DOM-Based Cross-Site Scripting. Variables should only be placed in a CSS property value. 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. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. If A is double JavaScript encoded then the following if check will return false. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. The line above could have possibly worked to render a link. Content Security Policy - An allowlist that prevents content being loaded. 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. The defined rules will HTML-escape < characters to prevent the creation of new HTML elements. However, depending on the tag which innerText is applied, code can be executed. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. For example, a JavaScript encoded string will execute even though it is JavaScript encoded. \u0061\u006c\u0065\u0072\u0074\u0028\u0037\u0037\u0029. What would be displayed in the input text field would be "Johnson & Johnson". If youre not using a framework or need to cover gaps in the framework then you should use an output encoding library. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. This variable includes some characters which are used in XSS attacks, namely <, " and >. 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. 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. These types of attacks typically occur as a result . 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. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. 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. Each encoder, Html, JavaScript and Url, must be configured separately. The reasoning behind this is to protect against unknown or future browser bugs (previous browser bugs have tripped up parsing based on the processing of non-English characters). Limit access to object properties when using object[x] accessors (Mike Samuel). A DOM-based XSS attack> is possible if the web application writes data to the Document Object Model without proper sanitization. Trusted Types are supported in Chrome 83, and a polyfill is available for other browsers. Read more about DOM-based cross-site scripting. The enterprise-enabled dynamic web vulnerability scanner. This can be done via a function such as: 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. This cheatsheet is a list of techniques to prevent or limit the impact of XSS. This helps quickly identify a large chunk of violations. In many cases, JavaScript encoding does not stop attacks within an execution context. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. This means, that no data will be available in server logs. One example of an attribute which is thought to be safe is innerText. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. Practise exploiting vulnerabilities on realistic targets. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. There will be situations where you use a URL in different contexts. DOM XSS in jQuery selector sink using a hashchange event, DOM XSS in AngularJS expression with angle brackets and double quotes HTML-encoded. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. So XSS has already been around for a while. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. The problem is that if companyName had the value "Johnson & Johnson". For example, this is the case if you're loading a third-party library from a CDN. HTML Validation (JSoup, AntiSamy, HTML Sanitizer). Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised. If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. HTML Context refers to inserting a variable between two basic HTML tags like a

or . 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. 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. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. 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. Now only JavaScript encoding on server side. Other CSS Contexts are unsafe and you should not place variable data in them. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. 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. 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. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Output encoding is not perfect. 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. OWASP are producing framework specific cheatsheets for React, Vue, and Angular. DOM-based XSS simply means a cross-site scripting vulnerability that occurs in the DOM ( Document Object Model) of your site rather than in HTML. If your code looked like the following, you would need to only double JavaScript encode input data. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts. 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). \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');",