How can you prevent cross-site scripting (XSS) vulnerabilities in a ColdFusion application?

Cross-site scripting (XSS) vulnerabilities in ColdFusion applications arise when user-supplied data is included in web pages without proper sanitization or encoding. An attacker can inject malicious scripts, typically JavaScript, into a website, which are then executed in the browsers of other users who visit the site. This allows the attacker to steal cookies, redirect users to malicious sites, or deface the website. Preventing XSS requires a layered approach focusing on input validation, output encoding, and utilizing ColdFusion's built-in security features.

First, input validation is crucial. This means verifying that user-supplied data conforms to expected formats and lengths *before* it's stored or used. For example, if a form field is intended for a postal code, validate that the input consists only of numbers and letters, and is within a reasonable length. ColdFusion's `isValid()` function and regular expressions can be used for this purpose. However, validation alone is not sufficient; it only prevents *obviously* malicious input, but clever attackers can bypass it.

Second, output encoding is the primary defense against XSS. Encoding transforms potentially dangerous characters into their safe equivalents before they are displayed in a web page. This prevents the browser from interpreting them as code. ColdFusion provides several encoding functions: `htmlFormat()`, `urlFormat()`, `javaScriptFormat()`, and `xmlFormat()`. The correct function depends on the context where the data is being displayed. `htmlFormat()` is the most commonly used, as it escapes characters like `<`, `>`, `&`, `