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Experimental.Sri Does Not Add Integrity To Inline Flight Scripts (Self.__Next_f.Push), Breaking Strict CSP Without 'Unsafe-Inline'

With experimental.sri enabled, Next.js App Router adds integrity attributes to external scripts but not to inline flight scripts, causing strict CSP policies without 'unsafe-inline' to block hydration and render client components non-functional.

highConfidence 85%Next.js Affected V16.2.9 Affected Vcanary

Origin Analysis

Next.js SubresourceIntegrityPlugin only processes webpack-emitted assets via compilation.getAssets(), so it cannot hash per-request inline flight scripts. The inline script tag creation in use-flight-response.ts constructs `<script>` or `<script nonce=...>` solely from a nonce, with no integrity property, because it has no access to the SRI manifest keyed by static filenames.

1. git clone https://github.com/bearpong/nextjs-sri-inline-csp-repro && cd nextjs-sri-inline-csp-repro 2. npm install 3. npm run build (SRI enabled via experimental.sri, strict CSP set via headers() — script-src 'self', no 'unsafe-inline', no nonce) 4. npm start (must be production build; next dev masks it) 5. Open http://localhost:3000 and open browser console. Observe repeated CSP violations for inline flight scripts; page does not hydrate.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This patch only adds the integrity attribute to inline scripts; it does not automatically add the corresponding sha256-* source to the Content-Security-Policy header. Users must still include the computed hashes in their CSP (or use a middleware to inject them) for browser execution. Additionally, hashing every inline script on each render adds CPU overhead and may affect streaming performance in high-throughput applications. Rollback: revert the patch to restore previous behavior (rebuild, no persistence). For concurrent rendering, ensure no global mutable hash caching is used; per-request computation is safe.