Vulnerability · Published Oct 8, 2026 · Assigned by GitHub_M

CVE-2026-107723fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array

High 8.1

In plain English

A high-severity security flaw in fast-jwt from nearform. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 40% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)8.1High

Exploit odds (30 days)0.5%top 60.3% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	fast-jwt	6.3.0	GHSA-5hjw-83fp-phq9

Affected products (as reported)

Vendor	Product	Versions
nearform	fast-jwt	< 6.3.0

Weakness: CWE-1287 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

References

https://github.com/nearform/fast-jwt/security/advisories/GHSA-5hjw-83fp-phq9 x_refsource_CONFIRM
https://github.com/nearform/fast-jwt/pull/639 x_refsource_MISC
https://github.com/nearform/fast-jwt/commit/86e83efd8b5244f50859532d99244f0a9a9a4368 x_refsource_MISC
https://github.com/nearform/fast-jwt/releases/tag/v6.3.0 x_refsource_MISC