{"id":"CVE-2026-16732","summary":"fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count","details":"fastify is a fast and low overhead web framework for Node.js. Impact: the fix for CVE-2026-3635 added a guard on the forwarded-header reads used to derive the request host, protocol, hostname, ip, and ips values, checking the connecting address. That guard closes the IP, CIDR, and custom-function forms of trustProxy correctly, because those forms compile to predicates that inspect the connecting address. The hop-count form, where trustProxy is set to a number, compiles to a predicate that structurally ignores the address, so the guard is always satisfied for any hop count of one or more. Applications configured with a numeric trustProxy value, such as trustProxy set to 1 for a single reverse proxy, remain vulnerable: an attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the forwarded request fields exactly as in the unpatched version. The impact class matches the parent CVE-2026-3635, including host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, and host-based routing and cache poisoning. Affected versions are fastify from 5.8.3 up to but not including 5.12.1. Patches: patched in fastify 5.12.1, where the numeric form of trustProxy is disabled at runtime and removed from the TypeScript type union. Workarounds: migrate to an IP, CIDR, or custom-function trustProxy value that validates the connecting address, and ensure the Fastify origin is only reachable through the trusted proxy chain.","aliases":["GHSA-3m5p-2c4r-xxw2"],"modified":"2026-09-04T03:47:19.649244610Z","published":"2026-08-18T20:30:22.592Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/16xxx/CVE-2026-16732.json","cna_assigner":"openjs","cwe_ids":["CWE-348"]},"references":[{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/16xxx/CVE-2026-16732.json"},{"type":"ADVISORY","url":"https://github.com/fastify/fastify/security/advisories/GHSA-3m5p-2c4r-xxw2"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-16732"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/fastify/fastify","events":[{"introduced":"a3e77ce171516e95f0c28ce60b9e5228a4c42fe4"},{"fixed":"7d196a998c422062a3aaa3f8041db91ad576cea0"}],"database_specific":{"extracted_events":[{"introduced":"5.8.3"},{"fixed":"5.12.1"}],"source":["AFFECTED_FIELD","CPE_RANGE"],"cpe":"cpe:2.3:a:fastify:fastify:*:*:*:*:*:node.js:*:*"}}],"versions":["v5.12.0","v5.11.3","v5.11.2","v5.11.1","v5.11.0","v5.10.0","v5.9.0","v5.8.5","v5.8.4","v5.8.3"],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-16732.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N"}]}