diff --git a/src/__mocks__/astro-middleware.ts b/src/__mocks__/astro-middleware.ts
new file mode 100644
index 0000000..afa1536
--- /dev/null
+++ b/src/__mocks__/astro-middleware.ts
@@ -0,0 +1,4 @@
+/** Minimal stub for astro:middleware used in Vitest only. */
+export function defineMiddleware(fn: unknown) {
+  return fn;
+}
diff --git a/src/middleware.test.ts b/src/middleware.test.ts
new file mode 100644
index 0000000..5cea4db
--- /dev/null
+++ b/src/middleware.test.ts
@@ -0,0 +1,52 @@
+import { describe, it, expect } from 'vitest';
+import { buildSecurityHeaders } from './middleware';
+
+describe('buildSecurityHeaders', () => {
+  const headers = buildSecurityHeaders();
+
+  it('sets a strict Content-Security-Policy', () => {
+    const csp = headers['Content-Security-Policy'];
+    expect(csp).toContain("default-src 'self'");
+    expect(csp).toContain("script-src 'self'");
+    expect(csp).toContain("frame-ancestors 'none'");
+    expect(csp).toContain("form-action 'none'");
+    expect(csp).toContain("base-uri 'self'");
+    expect(csp).toContain("object-src 'none'");
+  });
+
+  it('denies framing', () => {
+    expect(headers['X-Frame-Options']).toBe('DENY');
+  });
+
+  it('disables MIME sniffing', () => {
+    expect(headers['X-Content-Type-Options']).toBe('nosniff');
+  });
+
+  it('sets a strict referrer policy', () => {
+    expect(headers['Referrer-Policy']).toBe('strict-origin-when-cross-origin');
+  });
+
+  it('disables sensitive browser features', () => {
+    const pp = headers['Permissions-Policy'];
+    expect(pp).toContain('geolocation=()');
+    expect(pp).toContain('microphone=()');
+    expect(pp).toContain('camera=()');
+    expect(pp).toContain('payment=()');
+  });
+
+  it('sets HSTS with long max-age, subdomains, and preload', () => {
+    const hsts = headers['Strict-Transport-Security'];
+    expect(hsts).toContain('max-age=31536000');
+    expect(hsts).toContain('includeSubDomains');
+    expect(hsts).toContain('preload');
+  });
+
+  it('does not allow inline scripts', () => {
+    expect(headers['Content-Security-Policy']).not.toContain("'unsafe-inline' 'nonce-");
+    const scriptDirective = headers['Content-Security-Policy']
+      .split(';')
+      .map(s => s.trim())
+      .find(s => s.startsWith('script-src')) ?? '';
+    expect(scriptDirective).not.toContain("'unsafe-inline'");
+  });
+});
diff --git a/src/middleware.ts b/src/middleware.ts
new file mode 100644
index 0000000..f390a45
--- /dev/null
+++ b/src/middleware.ts
@@ -0,0 +1,51 @@
+import { defineMiddleware } from 'astro:middleware';
+
+/**
+ * Emits the site-wide security header set. Astro does not attach these to
+ * statically-built responses at runtime on a shared host — but `preview` uses
+ * them locally, and when Cloudflare Transform Rules are configured (per
+ * docs/superpowers/specs/2026-04-24-cameleer-website-design.md §5.3) the
+ * edge re-emits the same set for the prod origin. Having both is defense
+ * in depth.
+ */
+export function buildSecurityHeaders(): Record<string, string> {
+  const csp = [
+    "default-src 'self'",
+    "img-src 'self' data:",
+    "style-src 'self' 'unsafe-inline'",
+    "font-src 'self'",
+    "script-src 'self'",
+    "connect-src 'self'",
+    "frame-ancestors 'none'",
+    "base-uri 'self'",
+    "form-action 'none'",
+    "object-src 'none'",
+  ].join('; ');
+
+  const permissionsPolicy = [
+    'geolocation=()',
+    'microphone=()',
+    'camera=()',
+    'payment=()',
+    'usb=()',
+    'fullscreen=(self)',
+  ].join(', ');
+
+  return {
+    'Content-Security-Policy': csp,
+    'X-Frame-Options': 'DENY',
+    'X-Content-Type-Options': 'nosniff',
+    'Referrer-Policy': 'strict-origin-when-cross-origin',
+    'Permissions-Policy': permissionsPolicy,
+    'Strict-Transport-Security': 'max-age=31536000; includeSubDomains; preload',
+  };
+}
+
+export const onRequest = defineMiddleware(async (_context, next) => {
+  const response = await next();
+  const headers = buildSecurityHeaders();
+  for (const [name, value] of Object.entries(headers)) {
+    response.headers.set(name, value);
+  }
+  return response;
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index 7eeb3f8..157cceb 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -1,6 +1,12 @@
 import { defineConfig } from 'vitest/config';
+import path from 'path';
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      'astro:middleware': path.resolve('./src/__mocks__/astro-middleware.ts'),
+    },
+  },
   test: {
     environment: 'node',
     include: ['src/**/*.test.ts'],