Implementing CSP Effectively

Successful CSP implementation requires careful planning and gradual rollout. Start by implementing CSP in report-only mode using the Content-Security-Policy-Report-Only header. This allows you to collect violation reports without breaking functionality, identifying all the resources your application uses. Analyze these reports to understand your application's requirements and identify any unexpected resource usage that might indicate vulnerabilities or unnecessary dependencies.

Begin with a restrictive policy and gradually add exceptions as needed. A strict starting point might be: default-src 'none'; script-src 'self'; style-src 'self'; img-src 'self'; font-src 'self'; connect-src 'self'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'. This policy blocks inline scripts and styles, which prevents most XSS attacks but requires refactoring code that uses inline event handlers or style attributes. While this refactoring requires effort, it significantly improves your application's security posture.

Dealing with inline scripts and styles represents the biggest challenge in CSP implementation. The 'unsafe-inline' directive allows inline content but largely negates CSP's XSS protection. Instead, use nonces or hashes to allow specific inline content. Nonces require generating a unique random value for each response and including it in both the CSP header and script tags. Hashes involve computing cryptographic hashes of inline scripts and including these in the CSP. Both approaches allow necessary inline content while maintaining protection against injected scripts.