CVE-2026-3635: Fastify request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function
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.
Other sources
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.
— NVD
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-3635?
The severity of CVE-2026-3635 is classified as medium due to potential spoofing of request protocol and host.
How do I fix CVE-2026-3635?
To fix CVE-2026-3635, ensure the trustProxy setting is configured correctly to prevent accepting untrusted IP addresses.
Which versions of Fastify are affected by CVE-2026-3635?
CVE-2026-3635 affects Fastify versions up to and including 5.8.2.
What happens if CVE-2026-3635 is exploited?
If CVE-2026-3635 is exploited, untrusted connections may spoof the request protocol and host, leading to security vulnerabilities.
Is there a workaround for CVE-2026-3635?
A potential workaround for CVE-2026-3635 is to avoid using overly restrictive trust functions in trustProxy that may allow spoofing.