The Razor engine used in MVC automatically encodes all output sourced from variables, unless you work really hard to prevent it doing so. Any variable that does not go through this process is a potential weakness. 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. 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. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. 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. Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). 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. Policies are factories for Trusted Types that enforce certain security rules on their input: This code creates a policy called myEscapePolicy that can produce TrustedHTML objects via its createHTML() function. For example: The preceding markup generates the following HTML: The preceding code generates the following output: Do NOT concatenate untrusted input in JavaScript to create DOM elements or use document.write() on dynamically generated content. Its easy to make mistakes with the implementation so it should not be your primary defense mechanism. Sometimes users need to author HTML. Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. Each encoder, Html, JavaScript and Url, must be configured separately. Cross-site scripting (XSS) vulnerabilities occur when: Untrusted data enters a web application, typically from a web request. Consider adopting the following controls in addition to the above. When other users load affected pages the attacker's scripts will run, enabling the attacker to steal cookies and session tokens, change the contents of the web page through DOM manipulation or redirect the browser to another page. The rendered output would now become. Get the latest content on web security in your inbox each week. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. There will be situations where you use a URL in different contexts. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. Document Object Model (DOM) Based XSS. The line above could have possibly worked to render a link. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. For each potential source, such as location, you first need to find cases within the page's JavaScript code where the source is being referenced. The enterprise-enabled dynamic web vulnerability scanner. In order to understand DOM based XSS, one needs to see the fundamental difference between Reflected and Stored XSS when compared to DOM based XSS. 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). The other alternative is using N-levels of encoding. The primary difference is where the attack is injected into the application. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. 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]= It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). There are 3 primary types of cross-site scripting: DOM-based XSS. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. 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. 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. 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. 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. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. What would be displayed in the input text field would be "Johnson & Johnson". Use one of the following approaches to prevent code from being exposed to DOM-based XSS: createElement () and assign property values with appropriate methods or properties such as node.textContent= or node.InnerText=. For instance, jQuery's attr() function can change the attributes of DOM elements. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. your framework), you should be able to mitigate all XSS vulnerabilities. Variables should not be interpreted as code instead of text. There are several methods and attributes which can be used to directly render HTML content within JavaScript. Encode all characters with the %HH encoding format. 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. Encode all characters using the \xHH format. If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. HTML Attribute Contexts refer to placing a variable in an HTML attribute value. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. For example; If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. 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. However, depending on the tag which innerText is applied, code can be executed. 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. \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');",