CVE-2026-3635
This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by the application.
Does this matter?
Lower severity and a low EPSS score (0.12%). Track it; it rarely justifies an emergency change on its own.
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.
- CVSS 3.1
- 6.1 MEDIUMCVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
- EPSS
- 0.12% probability · 2th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-348
- Affected
- fastify/fastify
- Source
- ce714d77-add3-4f53-aff5-83d477b104bb
References
- https://cna.openjsf.org/security-advisories.htmlThird Party Advisory
- https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xfVendor Advisory
- https://www.cve.org/CVERecord?id=CVE-2026-3635Third Party Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.