GHSA-3m5p-2c4r-xxw2

## Impact

The fix for [CVE-2026-3635](https://www.cve.org/CVERecord?id=CVE-2026-3635) ([GHSA-444r-cwp2-x5xf](https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf)) added a `proxyFn(socket.remoteAddress, 0)` guard on the `X-Forwarded-*` reads in `request.host`, `request.protocol`, `request.hostname`, `request.ip`, and `request.ips`. That guard closes the IP, CIDR, and custom-function forms of `trustProxy` correctly because those forms compile to predicates that inspect the connecting address. The hop-count form (`trustProxy: <number>`) compiles to a predicate that structurally ignores the address argument, so the guard reduces to `0 < tp`, always true for any `tp >= 1`.

Applications configured with `trustProxy: <number>` (documented as "behind N reverse proxies", `trustProxy: 1` being the canonical single-proxy setting) remain vulnerable. An attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the request fields exactly as in the unpatched version. Impact class matches the parent CVE-2026-3635: host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie / CSRF-origin bypass, host-based routing and cache poisoning.

## Patches

Patched in fastify 5.12.1. The numeric form of `trustProxy` is now disabled at runtime and removed from the TypeScript type union.

## Workarounds

- Migrate to an IP / CIDR / custom-function `trustProxy` value that validates the connecting address. Custom functions must inspect the `address` argument, not only the hop index. - Ensure the Fastify origin is only reachable through the trusted proxy chain (no direct network path).