Introduction

A Content Security Policy can improve security and still break a site the moment it goes live. When a required script, stylesheet, font, frame, or API endpoint is not allowed by the current policy, browsers quietly refuse to load it. The result often looks like a front-end bug even though the server keeps returning 200 responses. The fix is to identify the exact blocked resource and update the policy with precision instead of weakening it broadly.

Symptoms

  • JavaScript features stop working after a header or CDN change
  • Fonts, styles, or embeds fail to load even though the files exist
  • Browser console logs show Content Security Policy violations
  • Only one environment breaks because staging and production use different resource origins
  • The issue started after enabling a stricter security header or third-party tool

Common Causes

  • The CSP does not include a required source for scripts, styles, fonts, frames, or network requests
  • A CDN, asset hostname, or third-party service was changed without updating the policy
  • Inline scripts or styles no longer match the current nonce, hash, or policy model
  • Multiple layers send competing CSP headers and the stricter one wins
  • The team loosened one directive previously, then forgot to carry it into a new deployment

Step-by-Step Fix

  1. Open the browser console and identify the exact CSP violation message, including the blocked resource and directive name.
  2. Compare the blocked resource origin to the current policy so you can see what is missing or overly strict.
  3. Check whether the site sends one CSP header or several from different layers such as the app, reverse proxy, or CDN.
  4. Verify any nonces, hashes, or inline script allowances still match the code generated by the current deployment.
  5. Review recent changes to asset hosts, tag managers, embeds, analytics, fonts, or API origins that may require policy updates.
  6. Update only the directive that needs expansion instead of broadly allowing unsafe sources across the whole policy.
  7. Re-test the broken page and confirm the blocked resources now load without introducing unrelated policy regressions.
  8. If third-party scripts are no longer needed, remove them rather than expanding the CSP to keep legacy integrations alive.
  9. Keep the policy and front-end dependencies versioned together so future deployments do not break security headers unexpectedly.