CVE-2026-3635: fastify Use of Less Trusted Source

Vulnerability summary

CVE-2026-3635 is a medium (CVSS 6.1) Use of Less Trusted Source vulnerability in fastify. EPSS estimates a 0.17% chance of exploitation in the next 30 days, higher than 5.7% of scored vulnerabilities.

Timeline

1. 23-03-2026
   PublishedVulnerability first disclosed.
2. 23-03-2026
   Last modifiedVulnerability information updated.

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.

• CWE-348Use of Less Trusted SourceThe product has two different sources of the same data or information, but it uses the source that has less support for verification, is less trusted, or is less resistant to attack.