{"id":"CVE-2026-76169","aliases":["GHSA-p68q-wchp-6fh7"],"url":"https://o3.security/vulnerability/CVE-2026-76169","summary":"fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers","details":"### Impact\n\nFastify routes a malformed URL under one plugin prefix to the custom not-found handler of a different sibling plugin, invoking the handler registered last and skipping the `preHandler` declared in its `setNotFoundHandler()`. When the request method has no route in the main router, a malformed request target reaches Fastify's internal not-found router before URL decoding and is dispatched through a single shared handler pointer, regardless of prefix and without the normal request lifecycle. An unauthenticated request to a public prefix can therefore reach an authentication-protected not-found handler registered under a different prefix and receive its full response, breaking prefix encapsulation and bypassing the authentication hook. Applications whose private or tenant fallbacks return protected data from a not-found handler are affected.\n\n### Patches\n\nPatched in fastify 5.12.2. Malformed URLs are now routed through the configured `onBadUrl` and `onMaxParamLength` handlers so they fail closed before any application not-found handler runs, and the shared not-found handler pointer has been removed.\n\n### Workarounds\n\nReject malformed request targets before they reach the application, for example at an upstream proxy or gateway, and do not rely on a not-found handler to serve protected data. A global `onRequest` authentication hook does not mitigate this, because the malformed-URL path skips it.","published":"2026-09-04T09:51:03.556Z","modified":"2026-10-21T02:59:40.292355286Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"npm","name":"fastify","fixedVersion":"5.12.2"}],"fix":{"url":"https://github.com/fastify/fastify/commit/93c239a380f3f2778bb7565fcdf777e91cfe1fbc","label":"fastify/fastify@93c239a"},"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"},{"type":"WEB","url":"https://github.com/fastify/fastify/commit/93c239a380f3f2778bb7565fcdf777e91cfe1fbc"},{"type":"PACKAGE","url":"https://github.com/fastify/fastify"},{"type":"WEB","url":"https://github.com/fastify/fastify/releases/tag/v5.12.2"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:59:40.292355286Z"}}