{"id":"CVE-2026-3635","aliases":["GHSA-444r-cwp2-x5xf"],"url":"https://o3.security/vulnerability/CVE-2026-3635","summary":"Fastify request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function","details":"## Summary\n\nWhen `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.\n\n## Affected Versions\n\nfastify <= 5.8.2\n\n## Impact\n\nApplications 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.\n\nWhen `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":"2026-03-23T13:53:00.386Z","modified":"2026-08-12T03:51:44.304994797Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"fastify","fixedVersion":"5.8.3"}],"fix":null,"references":[{"type":"WEB","url":"https://www.cve.org/CVERecord?id=CVE-2026-3635"},{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/3xxx/CVE-2026-3635.json"},{"type":"ADVISORY","url":"https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-3635"},{"type":"PACKAGE","url":"https://github.com/fastify/fastify"},{"type":"WEB","url":"https://github.com/fastify/fastify/releases/tag/v5.8.3"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:44.304994797Z"}}