CVE-2026-3635 fastify: request.protocol and request.host Spoofable via X-Forwarded-Proto/Host from Untrusted Connections
Attackers can spoof protocol and host headers in Fastify when restrictive trustProxy settings are used, bypassing security checks like HTTPS enforcement.
- 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-2026-3635?
CVE-2026-3635 is a medium-severity vulnerability in fastify, published on 23 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
Summary When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), the request.protocol and request.host getters read X-Forwarded-Proto and X-Forwarded-Host headers from any connection — including connections from untrusted IPs. This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by the application.
Affected Versions fastify <= 5.8.2
Impact Applications using request.protocol or request.host for security decisions (HTTPS enforcement, secure cookie flags, CSRF origin checks, URL construction, host-based routing) are affected when trustProxy is configured with a restrictive trust function.
When trustProxy: true (trust everything), both host and protocol trust all forwarded headers — this is expected behavior. The vulnerability only manifests with restrictive trust configurations.
Weakness: CWE-348
Which versions are affected by CVE-2026-3635?
2 open source packages ship an affected version, including fastify. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fastify npm | All versions | 5.8.3 |
fastify npm | All versions | 5.8.3 |
Products include Fastify Fastify.
How do I fix CVE-2026-3635?
Upgrade fastify to 5.8.3 or later.
Is CVE-2026-3635 being exploited?
There is no known exploitation of CVE-2026-3635 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 CLSA-2026-1786387037, GHSA-444r-cwp2-x5xf, MINI-546q-564w-xhxv.