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.
References
TinkAnet Reporter
climba03003 Remediation reviewer
mcollina Remediation reviewer
UlisesGascon Remediation reviewer
Summary
When
trustProxyis configured with a restrictive trust function (e.g., a specific IP liketrustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), therequest.protocolandrequest.hostgetters readX-Forwarded-ProtoandX-Forwarded-Hostheaders 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.protocolorrequest.hostfor security decisions (HTTPS enforcement, secure cookie flags, CSRF origin checks, URL construction, host-based routing) are affected whentrustProxyis configured with a restrictive trust function.When
trustProxy: true(trust everything), bothhostandprotocoltrust all forwarded headers — this is expected behavior. The vulnerability only manifests with restrictive trust configurations.References