Content Security Policy tells the browser which content sources are allowed. Its strongest value is limiting what injected markup can execute, especially when scripts require nonces or hashes.
Start from the real page
List first-party scripts, styles, fonts, images, frames and connections. Remove unused third parties before writing directives; a policy should describe intentional dependencies.
Use report-only mode
Deploy Content-Security-Policy-Report-Only, collect violations and separate real requirements from browser extensions and noise. Then enforce gradually.
Avoid the weak shortcut
unsafe-inline allows broad inline execution and can remove much of CSP's XSS value. Prefer nonces or hashes and generate nonces per response.
Test complete journeys
Login, payment, media, consent tools and error pages may load different resources. CSP is a maintained control, not a one-time header copied from a scanner.
Reports may contain sensitive URLs
CSP violation reports can include document and blocked-resource addresses. Send them to a controlled endpoint, limit retention and avoid placing secrets in URLs. Sample high-volume noise rather than storing everything indefinitely.
Third-party scripts remain trusted code
Allowlisting a vendor permits its script under the policy. Review whether the integration is still required and use Subresource Integrity where appropriate; CSP cannot make a compromised allowed script harmless.
Begin with an asset inventory
List first-party scripts, styles, images, fonts, frames and network endpoints. Remove obsolete third parties before building an allowlist. Start with Content-Security-Policy-Report-Only, collect violations in a controlled system and distinguish real dependencies from injected browser extensions.
Prefer nonces or hashes over broad exceptions
Host allowlists cannot protect against every compromised allowed source, while unsafe-inline weakens script protection substantially. Generate a fresh nonce per response for authorized inline scripts and avoid placing it in cached HTML incorrectly. Move event handlers and inline code into reviewed files where practical.
CSP is a mitigation, not a sanitizer. Continue context-aware output encoding, input validation and safe DOM APIs. Test authentication, payments, error pages and mobile layouts before enforcement.
Build a policy from observed dependencies
List where scripts, styles, images, fonts, frames and network requests actually originate. Start with default-src 'self', then open specific resource types only as needed. Hostnames should be exact enough that a compromise at an unrelated subdomain cannot satisfy the policy.
Inline scripts are the difficult part. Prefer external files or nonce-based authorization generated separately for each response. A static nonce, broad wildcard or 'unsafe-inline' restores much of the execution surface that CSP is meant to reduce.
Roll out without hiding breakage
Use Content-Security-Policy-Report-Only first and exercise login, checkout, forms, media and error states. Reports are signals, not proof of an attack. Browser extensions and injected software can create noise, so group violations by directive, blocked source and page.
Move enforced directives gradually and keep report-only rules stricter for the next improvement. Test the actual response after CDN changes. CSP is valuable defense in depth, but output encoding, safe DOM APIs and dependency control remain essential.
Keep a rollback that restores the previous known policy without disabling CSP completely. A small set of test pages helps distinguish a new application dependency from hostile injected code during an incident.