fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set

{
  "id": "1bd4bb0d3968d1fc6d0d16a357c5adcf79aedbc5ee6f7e5ce61ddaaeacb2bd50",
  "url": "https://osv.dev/vulnerability/GHSA-8wpc-h4q6-8fxv",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set",
    "summary": "### Summary `createVerifier` in fast-jwt ≤ 6.3.0 skips signature verification entirely when the `key` option is a falsy synchronous value (`''` or `null`) **and** the `algorithms` option is set to a non-empty allowlist. An attacker who can present a JWT to the application — regardless of algorithm — can forge arbitrary claims without possessing any signing key. ### Details **Root cause — four cooperating code paths:** **A. Falsy sync keys bypass `prepareKeyOrSecret`** `createVerifier` branches on `typeof key`: ```js const keyType = typeof key if (keyType !== 'string' && keyType !== 'object' && keyType !== 'function') { throw new TokenError(/* ... */) } if (key && keyType !== 'function') { key = prepareKeyOrSecret(key, hsAlgorithms.includes(availableAlgorithms[0])) } ``` When `key` is `''` (string, falsy) or `null` (object, falsy) the outer type check passes but the `if (key && ...)` guard never calls `prepareKeyOrSecret`. The empty-secret rejection added in that function is therefore never reached for sync keys: ```js function prepareKeyOrSecret(key, isSecret) { if (isSecret && key.length === 0) { throw new TokenError(TokenError.codes.invalidKey, 'The key cannot be an empty string or buffer.') } return isSecret ? createSecretKey(key) : createPublicKey(key) } ``` **B. Explicit `algorithms` keeps an allowlist active with no key** When `key` is falsy, autodetection is skipped and the caller-supplied `algorithms` (e.g. `['HS256']`) is retained in `allowedAlgorithms`. The verifier",
    "evidence": "osv:GHSA-8wpc-h4q6-8fxv",
    "products": [
      {
        "name": {
          "value": "fast-jwt",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.3.1",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "GHSA-8wpc-h4q6-8fxv",
      "aliases": [
        "CVE-2026-107720",
        "GHSA-8wpc-h4q6-8fxv"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "6.3.1"
                }
              ]
            }
          ],
          "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-8wpc-h4q6-8fxv/GHSA-8wpc-h4q6-8fxv.json",
            "last_known_affected_version_range": "<= 6.3.0"
          }
        }
      ],
      "modified": "2026-10-08T22:15:07.105532249Z",
      "provider": "osv",
      "references": [
        "https://github.com/nearform/fast-jwt/security/advisories/GHSA-8wpc-h4q6-8fxv",
        "https://github.com/nearform/fast-jwt/pull/649",
        "https://github.com/nearform/fast-jwt/commit/e22a151e83bf6d5e54e281216982d204d5665534",
        "https://github.com/nearform/fast-jwt",
        "https://github.com/nearform/fast-jwt/releases/tag/v6.3.1"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-20",
          "CWE-347"
        ],
        "severity": "HIGH",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-08T22:02:12Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-107720",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 104814,
  "published_at": "2026-10-08T22:02:12+00:00"
}