How to Fix "Refused to execute inline script because it violates Content Security Policy" (Complete 2026 Guide)

1. Root Cause Analysis (Engine-Level Breakdown)

When modern JavaScript engines (V8 in Chrome/Node.js, JavaScriptCore in Safari, and SpiderMonkey in Firefox) encounter this failure condition, execution halts or falls back to degraded behavior due to the following primary triggers:

• Inline `<script>` tag or inline event handler (`onclick=...`) executed without a cryptographic nonce matching the server CSP response header.
• Framework SSR HTML hydration injecting inline JSON state blobs without safe serialized script tags or nonces.
• Using `unsafe-inline` while also declaring a nonce or hash (modern browsers ignore `unsafe-inline` when a nonce or hash is present).

In high-scale production systems, this error rarely occurs during local development because local environments lack network latency, third-party browser extensions, complex caching proxies, and production minification transforms that uncover timing race conditions.

2. Verified Production Solutions

The following code recipes provide immediate and architectural fixes for this error:

// Next.js Middleware Nonce Generation (Next.js 14 & 15 App Router):
import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  const nonce = Buffer.from(crypto.randomUUID()).toString('base64');
  const cspHeader = `
    default-src 'self';
    script-src 'self' 'nonce-${nonce}' 'strict-dynamic' https: 'unsafe-inline';
    style-src 'self' 'unsafe-inline';
    img-src 'self' blob: data: https:;
    connect-src 'self' wss: https:;
    font-src 'self';
    object-src 'none';
    base-uri 'self';
    form-action 'self';
    frame-ancestors 'none';
    upgrade-insecure-requests;
  `.replace(/\s{2,}/g, ' ').trim();

  const requestHeaders = new Headers(request.headers);
  requestHeaders.set('x-nonce', nonce);
  requestHeaders.set('Content-Security-Policy', cspHeader);

  const response = NextResponse.next({ request: { headers: requestHeaders } });
  response.headers.set('Content-Security-Policy', cspHeader);
  return response;
}

3. Step-by-Step Resolution Workflow

Follow this structured checklist to resolve and prevent this error in your CI/CD pipeline: