{"id":"CVE-2026-33804","aliases":["GHSA-v9ww-2j6r-98q6"],"url":"https://o3.security/vulnerability/CVE-2026-33804","summary":"@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option","details":"### Impact\n\n`@fastify/middie` v9.3.1 and earlier does not read the deprecated (but still functional) top-level `ignoreDuplicateSlashes` option, only reading from `routerOptions`. This creates a normalization gap: Fastify's router normalizes duplicate slashes but middie does not, allowing middleware bypass via URLs with duplicate leading slashes (e.g., `//admin/secret`).\n\nThis only affects applications using the deprecated top-level configuration style (`fastify({ ignoreDuplicateSlashes: true })`). Applications using `routerOptions: { ignoreDuplicateSlashes: true }` are not affected.\n\nThis is distinct from [GHSA-8p85-9qpw-fwgw](https://github.com/fastify/middie/security/advisories/GHSA-8p85-9qpw-fwgw) (CVE-2026-2880), which was patched in v9.2.0.\n\n### Patches\n\nUpgrade to `@fastify/middie` >= 9.3.2.\n\n### Workarounds\n\nMigrate from deprecated top-level `ignoreDuplicateSlashes: true` to `routerOptions: { ignoreDuplicateSlashes: true }`.","published":"2026-04-16T13:56:56.176Z","modified":"2026-08-12T03:51:26.676677207Z","cvss":{"score":7.4,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"@fastify/middie","fixedVersion":"9.3.2"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33804.json"},{"type":"ADVISORY","url":"https://github.com/fastify/middie/security/advisories/GHSA-v9ww-2j6r-98q6"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33804"},{"type":"PACKAGE","url":"https://github.com/fastify/middie"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:26.676677207Z"}}