diff --git a/app/routes.ts b/app/routes.ts
index 6a2d3786..d549ea8e 100644
--- a/app/routes.ts
+++ b/app/routes.ts
@@ -176,4 +176,5 @@ export default [
       route('users', 'features/platform-admin/routes/users.tsx'),
     ]),
   ]),
+  route('*', 'shell/not-found.tsx'),
 ] satisfies RouteConfig
diff --git a/app/shell/not-found.test.ts b/app/shell/not-found.test.ts
new file mode 100644
index 00000000..a74393e0
--- /dev/null
+++ b/app/shell/not-found.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest'
+
+import { action, loader } from './not-found'
+
+describe('not-found route', () => {
+  it.each([
+    ['loader', loader],
+    ['action', action],
+  ])('answers 404 from its %s', (_name, handler) => {
+    let thrown: unknown
+    try {
+      handler()
+    } catch (error) {
+      thrown = error
+    }
+
+    expect(thrown).toBeInstanceOf(Response)
+    expect((thrown as Response).status).toBe(404)
+  })
+})
diff --git a/app/shell/not-found.tsx b/app/shell/not-found.tsx
new file mode 100644
index 00000000..432b5b20
--- /dev/null
+++ b/app/shell/not-found.tsx
@@ -0,0 +1,22 @@
+// Catch-all, registered last in app/routes.ts. When no route matches, React Router answers the 404
+// before any middleware runs, so the page left without the security headers (and its scripts
+// without the CSP nonce) that the root route gives every other response. Matching here runs the
+// root middleware and loader; the root ErrorBoundary still renders the page.
+function notFound(): never {
+  throw new Response(null, { status: 404, statusText: 'Not Found' })
+}
+
+export function loader() {
+  notFound()
+}
+
+// Without it, a form posted to an unknown path would answer 405 instead of 404.
+export function action() {
+  notFound()
+}
+
+// Never rendered. Without a component this would be a resource route, and the 404 would leave as
+// an empty response instead of the root ErrorBoundary's page.
+export default function NotFound() {
+  return null
+}
diff --git a/app/tests/e2e/not-found.spec.ts b/app/tests/e2e/not-found.spec.ts
new file mode 100644
index 00000000..aa7629eb
--- /dev/null
+++ b/app/tests/e2e/not-found.spec.ts
@@ -0,0 +1,21 @@
+import { expect, test } from '@playwright/test'
+
+test.describe('Unknown path', () => {
+  // Regression: when no route matched, no middleware ran, so the 404 page left without the
+  // security headers every other response carries.
+  test('answers 404 with the security headers', async ({ request }) => {
+    const response = await request.get('/this-path-does-not-exist')
+
+    expect(response.status()).toBe(404)
+    expect(response.headers()['x-frame-options']).toBe('DENY')
+    expect(response.headers()['x-content-type-options']).toBe('nosniff')
+    expect(response.headers()['content-security-policy-report-only']).toContain("default-src 'self'")
+  })
+
+  test('still renders the not-found page', async ({ page }) => {
+    const response = await page.goto('/this-path-does-not-exist')
+
+    expect(response?.status()).toBe(404)
+    await expect(page.getByText('404')).toBeVisible()
+  })
+})