{"id":"CVE-2026-3635","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-3635","generatedAt":"2026-10-27T23:57:40.467Z","title":"This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by the application.","description":"Summary\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\nAffected Versions\nfastify <= 5.8.2\n\nImpact\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":"2026-03-23T14:16:35.000Z","lastModified":"2026-06-17T10:43:54.000Z","status":"Analyzed","sourceIdentifier":"ce714d77-add3-4f53-aff5-83d477b104bb","cvss":{"version":"3.1","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"},"cwe":["CWE-348"],"affected":["fastify/fastify"],"epss":{"score":0.00161,"percentile":0.05285,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"low","text":"Lower severity and a low EPSS score (0.16%). Track it; it rarely justifies an emergency change on its own."},"changes":[],"references":[{"url":"https://cna.openjsf.org/security-advisories.html","tags":["Third Party Advisory"],"source":"ce714d77-add3-4f53-aff5-83d477b104bb"},{"url":"https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf","tags":["Vendor Advisory"],"source":"ce714d77-add3-4f53-aff5-83d477b104bb"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-3635","tags":["Third Party Advisory"],"source":"ce714d77-add3-4f53-aff5-83d477b104bb"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-3635","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}