---
cve: "CVE-2026-16732"
severity: "MEDIUM"
cvss: 6.1
epss: "14%"
vendor: "fastify"
kev: false
exploited: false
published: "2026-08-18 21:16:34"
tags: [cve, security, medium]
source: tsecurity.de CVE-Dossier
exported: "2026-09-06T08:30:57+02:00"
---

# CVE-2026-16732

> 6.1 MEDIUM · 🧪 PoC

## Beschreibung

fastify is a fast and low overhead web framework for Node.js. Impact: the fix for CVE-2026-3635 added a guard on the forwarded-header reads used to derive the request host, protocol, hostname, ip, and ips values, checking the connecting address. 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, where trustProxy is set to a number, compiles to a predicate that structurally ignores the address, so the guard is always satisfied for any hop count of one or more. Applications configured with a numeric trustProxy value, such as trustProxy set to 1 for a single reverse proxy, remain vulnerable: an attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the forwarded request fields exactly as in the unpatched version. The impact class matches the parent CVE-2026-3635, including host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, and host-based routing and cache poisoning. Affected versions are fastify from 5.8.3 up to but not including 5.12.1. Patches: patched in fastify 5.12.1, where the numeric form of trustProxy is disabled at runtime and removed from the TypeScript type union. Workarounds: migrate to an IP, CIDR, or custom-function trustProxy value that validates the connecting address, and ensure the Fastify origin is only reachable through the trusted proxy chain.

## CVSS-Vektor

```
CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Benachbart | warn |
| AC Komplexität | Hoch | good |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |
| S Scope | Verändert | bad |
| C Vertraulichkeit | Hoch | bad |
| I Integrität | Keine | good |
| A Verfügbarkeit | Keine | good |

## Patch verfügbar (OSV)

- 7d196a998c422062a3aaa3f8041db91ad576cea0 (Commit)

## Referenzen

- <https://github.com/fastify/fastify/security/advisories/GHSA-3m5p-2c4r-xxw2>
- <https://cna.openjsf.org/security-advisories.html>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-16732) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_