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Middleware Rewrites Are Network-Proxied When Using `Next Start -H 127.0.0.1`, Causing EPROTO 500 Behind TLS-Terminating Proxies

When Next.js is started with `-H 127.0.0.1` behind a reverse proxy that sets `X-Forwarded-Proto: https`, middleware rewrites targeting the same server are incorrectly treated as external and proxied over the network using the `https` scheme against the plain-HTTP port, resulting in `write EPROTO ... wrong version number` and a 500 response. The root cause is an asymmetric normalization of loopback hostnames between the router server's `initUrl` and the URL exposed to middleware.

highConfidence 85%Next.js Affected V16.2.6 Affected V16.3.0-Canary.49

Origin Analysis

The router server builds `initUrl` using the literal value of `-H` (e.g., `127.0.0.1`) and the scheme from `x-forwarded-proto`, producing `https://127.0.0.1:3000` as the origin. In contrast, the URL available inside middleware goes through `NextURL.parseURL()`, which normalizes any loopback hostname (`127.0.0.0/8`, `::1`) to `localhost`. Thus, a rewrite from middleware using `new URL('/b', request.url)` yields `https://localhost:3000/b`. The subsequent `getRelativeURL` comparison only relativizes when origins exactly match, so `localhost` ≠ `127.0.0.1`, the rewrite remains absolute, and the router takes the external-proxy path. `proxyRequest` preserves the `https` scheme, leading to a TLS handshake attempt against the plain HTTP port and the observed EPROTO failure.

1. Clone or create a minimal Next.js app with a middleware file that rewrites `/a` to `/b`: ```ts export default function proxy(request: NextRequest) { if (request.nextUrl.pathname === '/a') { return NextResponse.rewrite(new URL('/b', request.url)); } } ``` 2. Run `npm install && npm run build`. 3. Start the server with an explicit loopback hostname: `npm start -- -H 127.0.0.1 -p 3000`. 4. Simulate a TLS-terminating reverse proxy by sending the `X-Forwarded-Proto: https` header: `curl -i -H 'X-Forwarded-Proto: https' http://127.0.0.1:3000/a`. 5. Observe the 500 response and server log containing `Failed to proxy https://localhost:3000/b Error: write EPROTO ... SSL routines:tls_validate_record_header:wrong version number`. 6. Repeating the request without `-H` or with `-H localhost` does not trigger the failure, confirming the loopback normalization asymmetry.

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

Testing should cover deployments using `-H 127.0.0.1` with and without `x-forwarded-proto`, as well as IPv6 loopback (`-H ::1`). The normalisation may alter the hostname used in internal logging or in rare cases where a self-rewrite intentionally targets a different loopback address; however, such usage is unlikely and already broken behind TLS proxies. Rollback: simply revert the change to restore previous behavior. Additional testing on Windows and with edge functions is recommended, as loopback address syntax and proxy behavior may vary.