CVE-2026-16732 fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count
Attackers can spoof forwarded headers to bypass security controls like HTTPS enforcement and CSRF protection when using numeric trustProxy settings.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-16732?
CVE-2026-16732 is a medium-severity vulnerability in fastify, published on 18 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Weakness: CWE-348
Which versions are affected by CVE-2026-16732?
2 open source packages ship an affected version, including fastify. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fastify npm | From 5.8.3 | 5.12.1 |
fastify npm | From 5.8.3 | 5.12.1 |
Products include Fastify Fastify.
How do I fix CVE-2026-16732?
Upgrade fastify to 5.12.1 or later.
Is CVE-2026-16732 being exploited?
There is no known exploitation of CVE-2026-16732 in the wild and no public exploit code.
Other vulnerabilities in fastify
- CVE-2026-84504 fastify vulnerable to request body replacement via an async validation result collision
- CVE-2026-76169 fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
- CVE-2026-84428 fastify vulnerable to header validation bypass via incomplete schema case normalization
- CVE-2026-84469 fastify vulnerable to request validation bypass via skipped boolean false schemas
- CVE-2026-33806 fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header
Sources
Also known as GHSA-3m5p-2c4r-xxw2, MINI-72wj-4m78-5w2p.