fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array

{
  "id": "914273c3b43058ab4f08ee56d4d161eec52d02a3ff0bc49e9268033f1e9fe2ac",
  "url": "https://osv.dev/vulnerability/GHSA-5hjw-83fp-phq9",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array",
    "summary": "### Summary `fast-jwt`'s `createVerifier` silently skips **all** configured claim validators (`exp`, `nbf`, `iss`, `aud`, `sub`, `jti`, `nonce`) when a validly-signed JWT carries a JSON array as its payload instead of an object. The verifier reports success while having enforced only the signature. This breaks the library's documented `allowedIss` / `allowedAud` / `allowedSub` / expiry / replay-protection guarantees and violates RFC 7519 §7.2 step 10, which requires the JWT Claims Set to be a JSON object. ### Details The decoder validates the **header** is a non-array object but the **payload** check is missing the `Array.isArray` guard: ```javascript // src/decoder.js:49 — header check (correct) if (!header || typeof header !== 'object' || Array.isArray(header)) { throw new TokenError(TokenError.codes.malformed, 'The token header is not a valid JSON object.') } // src/decoder.js:65 — payload check (vulnerable) if (!payload || typeof payload !== 'object') { // typeof [] === 'object' throw new TokenError(TokenError.codes.invalidPayload, 'The payload must be an object', { payload }) } Because typeof [] === 'object' in JavaScript, an array payload passes the decoder. In the verifier's validator loop, every check is short-circuited by an in-test that is always false for an array (arrays have only numeric indices and length): // src/verifier.js:304-323 for (const { type, claim, allowed, array, modifier, greater, errorCode, errorVerb } of validators) { const value = payload[claim] ",
    "evidence": "osv:GHSA-5hjw-83fp-phq9",
    "products": [
      {
        "name": {
          "value": "fast-jwt",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.3.0",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "GHSA-5hjw-83fp-phq9",
      "aliases": [
        "CVE-2026-107723",
        "GHSA-5hjw-83fp-phq9"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "6.3.0"
                }
              ]
            }
          ],
          "package": {
            "name": "fast-jwt",
            "purl": "pkg:npm/fast-jwt",
            "ecosystem": "npm"
          },
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-5hjw-83fp-phq9/GHSA-5hjw-83fp-phq9.json",
            "last_known_affected_version_range": "<= 6.2.4"
          }
        }
      ],
      "modified": "2026-10-08T22:15:05.904211357Z",
      "provider": "osv",
      "references": [
        "https://github.com/nearform/fast-jwt/security/advisories/GHSA-5hjw-83fp-phq9",
        "https://github.com/nearform/fast-jwt/pull/639",
        "https://github.com/nearform/fast-jwt/commit/86e83efd8b5244f50859532d99244f0a9a9a4368",
        "https://github.com/nearform/fast-jwt",
        "https://github.com/nearform/fast-jwt/releases/tag/v6.3.0"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-1287"
        ],
        "severity": "HIGH",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-08T22:01:58Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-107723",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 104811,
  "published_at": "2026-10-08T22:01:58+00:00"
}