{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "extracted_events": [
              {
                "introduced": "0"
              },
              {
                "fixed": "5.12.2"
              }
            ],
            "source": [
              "AFFECTED_FIELD",
              "DESCRIPTION"
            ]
          },
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "942a2be8704244db778f8db9a0bb4951b8825156"
            }
          ],
          "repo": "https://github.com/fastify/fastify",
          "type": "GIT"
        }
      ]
    }
  ],
  "aliases": [
    "GHSA-9q9j-q6p8-xq58"
  ],
  "database_specific": {
    "cna_assigner": "openjs",
    "cwe_ids": [
      "CWE-178"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84428.json"
  },
  "details": "fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.",
  "id": "CVE-2026-84428",
  "modified": "2026-09-10T03:30:19.679239003Z",
  "published": "2026-09-04T10:24:27.809Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://cna.openjsf.org/security-advisories.html"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84428.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/fastify/fastify/security/advisories/GHSA-9q9j-q6p8-xq58"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-84428"
    }
  ],
  "schema_version": "1.9.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fastify vulnerable to header validation bypass via incomplete schema case normalization"
}