CVE-2026-3635
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 fu…
Impact
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.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| npm | fastify | <= 5.8.2 | 5.8.3 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Fastify | Fastify | before 5.8.3 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-444r-cwp2-x5xf
- https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf
- https://nvd.nist.gov/vuln/detail/CVE-2026-3635
- https://cna.openjsf.org/security-advisories.html
- https://github.com/fastify/fastify/releases/tag/v5.8.3
- https://www.cve.org/CVERecord?id=CVE-2026-3635