{"id":"CVE-2026-76169","summary":"fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers","details":"fastify versions \u003e= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. An unauthenticated attacker can therefore reach an authentication-protected private fallback through an unrelated public prefix and read its full response, bypassing the authentication hook and breaking prefix encapsulation. Users should upgrade to fastify 5.12.2 or later.","aliases":["GHSA-p68q-wchp-6fh7"],"modified":"2026-09-10T03:30:58.937843297Z","published":"2026-09-04T09:51:03.556Z","database_specific":{"cwe_ids":["CWE-288"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/76xxx/CVE-2026-76169.json","cna_assigner":"openjs"},"references":[{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/76xxx/CVE-2026-76169.json"},{"type":"ADVISORY","url":"https://github.com/fastify/fastify/security/advisories/GHSA-p68q-wchp-6fh7"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-76169"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/fastify/fastify","events":[{"introduced":"e941c24bf51af7184cf658ca138e069e8c166ef8"},{"fixed":"942a2be8704244db778f8db9a0bb4951b8825156"}],"database_specific":{"extracted_events":[{"introduced":"4.0.0"},{"fixed":"5.12.2"},{"introduced":"0"}],"source":["AFFECTED_FIELD","DESCRIPTION"]}}],"versions":["v5.12.1","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","v5.8.2","v5.8.1","v5.8.0","v5.7.4","v5.7.3","v5.7.2","v5.7.1","v5.7.0","v5.6.2","v5.6.1","v5.6.0","v5.5.0","v5.4.0","v5.3.3","v5.3.2","v5.3.1","v5.3.0","v5.2.2","v5.2.1","v5.2.0","v5.1.0","v5.0.0","v5.0.0-alpha.4","v5.0.0-alpha.2","v5.0.0-alpha.1","v4.5.2","v4.5.1","v4.5.0","v4.4.0","v4.3.0","v4.2.1","v4.2.0","v4.1.0","v4.0.3","v4.0.2","v4.0.1","v4.0.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-76169.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"}]}