dom based cross site scripting prevention

There are two distinct groups of cross-site scripting. Cookie Attributes - These change how JavaScript and browsers can interact with cookies. 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. Reduce the DOM XSS attack surface of your application. WAFs are unreliable and new bypass techniques are being discovered regularly. A stored XSS attack enables an attacker to embed a malicious script into a vulnerable page, which is then executed when a victim views the page. For a comprehensive list, check out the DOMPurify allowlist. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. 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. WAFs are not recommended for preventing XSS, especially DOM-Based XSS. If you directly access an encoder via System.Text.Encodings.Web. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. 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. Cross-Site Scripting (XSS) is a misnomer. Use a nonce-based Content Security Policy for additional mitigation against the bugs as they inevitably happen. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. Products Insight Platform Solutions XDR & SIEM INSIGHTIDR Threat Intelligence THREAT COMMAND Vulnerability Management INSIGHTVM Dynamic Application Security Testing INSIGHTAPPSEC Make sure any attributes are fully quoted, same as JS and CSS. Semgrep rule to identify above dom xss link. There will be situations where you use a URL in different contexts. 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. Encode all characters using the \xHH format. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. This will solve the problem, and it is the right way to re-mediate DOM based XSS vulnerabilities. This type of attack is explained in detail in the following article: DOM XSS: An Explanation of DOM-based Cross-site Scripting. 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 logic which parses URLs in both execution and rendering contexts looks to be the same. This means you will need to use alternative elements like img or iframe. No single technique will solve XSS. Record your progression from Apprentice to Expert. Despite being rare, they may cause serious problems and only a few scanners can detect them. Please note, element.setAttribute is only safe for a limited number of attributes. XSS Prevention & Mitigation. Before putting untrusted data inside an HTML element ensure it's HTML encoded. You should apply HTML attribute encoding to variables being placed in most HTML attributes. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. 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. We want to hear from you! 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. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. The rendered output would now become. For example: To make dynamic updates to HTML in the DOM safe, we recommend: The HTML attribute subcontext within the execution context is divergent from the standard encoding rules. This can be done via a function such as: The DOM is a programming interface. Please refer to the list below for details. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. An alternative to using Element.setAttribute() to set DOM attributes is to set the attribute directly. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. 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. If your code looked like the following, you would need to only double JavaScript encode input data. 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. This behavior was often implemented using a vulnerable hashchange event handler, similar to the following: As the hash is user controllable, an attacker could use this to inject an XSS vector into the $() selector sink. This article looks at preventing Cross Site Scripting, a third common type of vulnerability in websites. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. 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. The only safe location for placing variables in JavaScript is inside a quoted data value. \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');",