EUVD-2026-95348
GHSA-5hjw-83fp-phq9
Severity by source
AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Signature is still enforced, so the attacker needs ability to influence a validly-signed token (PR:L) and a specific array-payload condition (AC:H); impact is full auth-control bypass affecting C and I, not availability.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
Lifecycle Timeline
5DescriptionCVE.org
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.
AnalysisAI
Signed JSON Web Tokens whose payload is a JSON array silently bypass every configured claim validator in the npm package fast-jwt through version 6.2.4, so the verifier returns success while enforcing only the signature and skipping exp, nbf, iss, aud, sub, jti and nonce checks. The flaw is exploitable by an authenticated attacker (CVSS PR:L) who can produce or influence a validly-signed token - signature verification itself is not broken, so the attacker must have access to the signing key or to a token-issuance path that lets them control the payload structure - and the true risk is gated by that precondition and by AC:H rather than the raw 8.1 base score. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Requires an application using fast-jwt createVerifier at version <=6.2.4, AND an attacker able to produce or influence a validly-signed token (signature verification is NOT bypassed, so this presumes access to the signing secret/key or a token-issuance path that permits controlling the payload structure). … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | This is a genuine high-impact authentication-logic flaw, but its real-world risk is gated by a non-trivial precondition that the raw CVSS 8.1 understates. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Upgrade to fast-jwt 6.3.0 or later, the vendor-released patch that adds the missing `Array.isArray(payload)` guard in src/decoder.js (see GHSA-5hjw-83fp-phq9, PR https://github.com/nearform/fast-jwt/pull/639 and commit 86e83efd8b5244f50859532d99244f0a9a9a4368). … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, inventory all services using fast-jwt through 6.2.4, confirm whether the opt-in requiredClaims option is enabled, enable it where absent, and reject any signed JWT whose payload is a JSON array; also confirm exposure against the publicly available exploit code and notify application owners. …
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External POC / Exploit Code
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