Understanding the Need for Content Security Policy

Modern web applications face constant threats from attackers attempting to inject malicious scripts into trusted websites. These attacks can steal user credentials, hijack sessions, deface websites, or turn legitimate sites into distribution points for malware. Traditional security measures like input validation and output encoding, while essential, can fail due to implementation errors or overlooked edge cases. CSP acts as a defense-in-depth mechanism, providing protection even when other security measures fail.

The fundamental principle behind CSP is simple yet powerful: instead of blindly trusting all content that appears to come from your server, the browser enforces a policy that explicitly defines which sources of content are legitimate. This approach transforms the browser from a potential accomplice in attacks into an active participant in your security strategy. When properly implemented, CSP can prevent the execution of unauthorized scripts, the loading of malicious stylesheets, and the establishment of unwanted connections to external servers.