CVE-2026-3635

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.

PUBLISHED Reserved 2026-03-06 | Published 2026-03-23 | Updated 2026-03-23 | Assigner openjs

Problem types

CWE-348 Use of less trusted source

Product status

Credits

LetaoZhao (TinkAnet) reporter

KaKa (climba03003) remediation reviewer

Matteo Collina remediation reviewer

Ulises Gascón remediation reviewer

References

github.com/...astify/security/advisories/GHSA-444r-cwp2-x5xf

www.cve.org/CVERecord?id=CVE-2026-3635

cna.openjsf.org/security-advisories.html

cve.org (CVE-2026-3635)

nvd.nist.gov (CVE-2026-3635)

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