Content Security Policy: Build a Policy Without Breaking the Site

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.