# Content Security Policy: Build a Policy Without Breaking the Site

> CSP reduces the impact of injected content, but copied policies often break legitimate scripts. Learn an inventory-first rollout.

- Canonical article: [https://www.metacyber.guru/articles/content-security-policy-basics](https://www.metacyber.guru/articles/content-security-policy-basics)
- Category: Website Security
- Author: Muhammad Azhar
- Published: August 14, 2026
- Last reviewed: 2026-08-14

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.

## Sources and further reading

- [MDN: Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CSP)
- [OWASP: CSP Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Content_Security_Policy_Cheat_Sheet.html)

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