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Forwarded Server Actions Expose Internal Origin (Localhost:PORT) Via Headers()

When a Server Action is forwarded to a worker that does not contain the action in its bundle, Next.js performs an internal HTTP fetch to `__NEXT_PRIVATE_ORIGIN` (http://localhost:PORT). This causes the forwarded subrequest's `Host` header to be derived from the URL, so `headers().get('host')` inside the action returns `localhost:PORT` instead of the original request's host. Self-hosted multi-tenant applications that resolve tenants from the `host` header break specifically for forwarded actions.

highConfidence 85%Next.js Affected V16.2.2

Origin Analysis

Forwarded Server Actions are sent via an internal `fetch` to `${__NEXT_PRIVATE_ORIGIN}${workerPathname}`, where `__NEXT_PRIVATE_ORIGIN` is set to `http://localhost:PORT` in `start-server.ts`. The `fetch` API derives the `Host` header from the URL and cannot be overridden because `Host` is a forbidden header in the Fetch spec. Although `x-forwarded-host` is preserved and carries the original host, `headers().get('host')` reads the derived `localhost:PORT` value.

1. Create a shared Server Action that reads `headers()` and returns `host`, `x-forwarded-host`, and `x-action-forwarded`. 2. Import the action only on one page (e.g., `/a`) and not on another (e.g., `/b`). 3. Build and start the app with `next build && next start -p 4111`. 4. POST the action to `/b` (which is not in the action's worker set) with a custom `Host` header. 5. Observe that the response contains `host: 'localhost:4111'` instead of the original host, while `x-forwarded-host` contains the original host and `x-action-forwarded` is `'1'`.

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

Mutating `req.headers.host` may impact any code that expects the internal host (`localhost:PORT`) during forwarded actions, such as logging or internal routing logic. Ensure that the `x-forwarded-host` header is trustworthy (e.g., set by a trusted reverse proxy); if not, this change could expose an attacker-controlled host to tenant resolution logic. Rollback: remove the added block and use `x-forwarded-host` directly in application code as a workaround. Test thoroughly with IPv6, multiple reverse proxies, and concurrent requests.