Content Security Policy (CSP) is a security mechanism implemented through an HTTP response header that allows website owners to control which resources (scripts, styles, images, fonts, etc.) the browser is permitted to load. CSP is a powerful defense against cross-site scripting (XSS) and data injection attacks.
A CSP header contains directives that specify allowed sources for different resource types. For example, script-src defines where JavaScript can be loaded from, style-src controls CSS sources, and img-src governs image sources. The default-src directive provides a fallback for any resource type not explicitly configured. Web application security books on Amazon provide implementation guides.
For URL shortening services, CSP helps protect splash pages, preview pages, and the management dashboard from XSS attacks. A well-configured CSP prevents attackers from injecting malicious scripts even if they find an input validation vulnerability.
Implementing CSP can be challenging because overly restrictive policies may break legitimate functionality. The recommended approach is to start with a report-only mode (Content-Security-Policy-Report-Only) that logs violations without blocking resources, then gradually tighten the policy based on the reports. Security engineering books on Amazon discuss deployment strategies.